Initial commit.

2026-08-31 15:45:58 +03:00
commit ec9805b01a
22 changed files with 1536 additions and 0 deletions
--- a/.gitignore
+++ b/.gitignore
@@ -0,0 +1,4 @@
+.pio
+.omo
+.junie
+.idea
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -0,0 +1,104 @@
+# AGENTS.md
+
+PlatformIO / Arduino-framework firmware for a NUCLEO-F042K6 reading two MS5611
+barometers (one I2C, one SPI) and emitting validated CSV over the ST-LINK VCP.
+See `README.md` for wiring tables, CSV format and status codes.
+
+## Commands
+
+```sh
+pio run                     # build (the only real verification gate)
+pio run -t upload           # flash over ST-LINK
+pio device monitor          # 115200 baud
+pio run -t clean
+```
+
+There is one env, `nucleo_f042k6`. No lint, format, or typecheck step exists.
+
+**Always read the size report at the end of `pio run`.** It is the acceptance
+criterion for any change (see below). Current: flash 88.8% (29108 / 32768),
+RAM 32.6% (2004 / 6144).
+
+## Flash budget is the dominant constraint
+
+3.6 KB of flash headroom. This shapes almost every design decision here, and it
+is the single easiest thing to break without noticing.
+
+`-flto` and `-fsingle-precision-constant` in `platformio.ini` are load-bearing.
+Verified: building without them overflows `FLASH` by **7252 bytes** and fails to
+link. Both MS5611 driver libraries write their compensation maths with unsuffixed
+double literals, which otherwise links the double soft-float helpers.
+
+Consequences for new code:
+
+- Never introduce `double`, unsuffixed floating literals, or `<math.h>` calls on
+  doubles. Use `float` and `f`-suffixed constants.
+- Never call `Serial.print(someFloat, digits)` — it resolves to
+  `Print::print(double, int)` and pulls in soft-float. `main.cpp::printFixed2()`
+  exists solely to format floats via scaled integers; use it.
+- Wrap string literals in `F()` so they stay in flash.
+- No `String`, no `new`/`delete`. `~BaroChannel()` is deliberately non-virtual
+  and `protected` to keep a destructor slot out of the vtable and avoid linking
+  `operator delete`. Do not "fix" this into a virtual destructor.
+- `lib_deps` pin exact driver tags (`MS5611#0.5.2`, `MS5611_SPI#0.4.3`). Bumping
+  them can blow the budget; rebuild and check size if you do.
+
+## Architecture
+
+- `include/config.h` — every pin, address, rate, threshold and validation limit.
+  Change hardware behaviour **here**, not in the `.cpp` files.
+- `include/baro_channel.h` / `src/baro_channel.cpp` — `BaroChannel`, the shared
+  state machine: init + PROM/CRC handshake, per-poll validation, error streaks,
+  re-init, and the 1 s error LED pulse. Five pure-virtual `driver*()` hooks are
+  the only per-bus surface.
+- `src/baro_i2c.cpp` / `src/baro_spi.cpp` — one concrete subclass each, plus a
+  file-static instance exposed through `baroI2cChannel()` / `baroSpiChannel()`.
+- `src/ms5611_crc.cpp` — standalone PROM CRC-4 (AN520).
+- `src/main.cpp` — `setup()`/`loop()`, CSV formatting, 10 Hz report scheduling.
+
+**The two-file split is mandatory, not stylistic.** `MS5611.h` and
+`MS5611_SPI.h` each define their own `enum osr_t` and `MS5611_READ_OK`, so they
+cannot be included in the same translation unit. `baro_channel.h` therefore
+includes neither driver header, and the channels are handed out via accessor
+functions. Do not merge these TUs or hoist a driver include into the header.
+
+## Conventions that differ from defaults
+
+- **millis() rollover safety**: deadline comparisons use a signed difference —
+  `if ((int32_t)(now - deadline) >= 0)`. Never write `now >= deadline`.
+- **`poll()` blocks** for ~2 ADC conversions (~5 ms/sensor at OSR 1024).
+  `main.cpp` re-reads `millis()` after polling before evaluating LED and report
+  deadlines. Preserve that if you touch the loop.
+- **Validation belongs in `BaroChannel`, not the drivers.** Neither library
+  checks the PROM CRC, and `MS5611::read()` returns `MS5611_READ_OK` for an ADC
+  read taken before the conversion completed (which yields 0). Do not simplify
+  the range / stale / all-zero-PROM checks away by trusting driver return codes.
+- Style is Allman braces, 2-space indent, `_camelCase` private members, `//  `
+  (two spaces) block comments. There is **no `.clang-format`** in the repo —
+  running `clang-format` would reformat everything into LLVM style. Don't.
+- `SPI` and `I2C` appear in `lib_deps`, but the code uses `Wire` directly and
+  nothing includes the third-party `I2C` library.
+
+## Testing
+
+There is no automated test suite. `test/` holds only the stock PlatformIO
+README, and `pio test` collects 0 cases. Do not claim tests pass.
+
+`ms5611_crc.{h,cpp}` is intentionally free of Arduino headers so it can be
+exercised on the host:
+
+```sh
+clang++ -std=c++17 -Wall -Wextra -c src/ms5611_crc.cpp -Iinclude -o /tmp/crc.o
+```
+
+The differential check against the NuttX reference described in `README.md` was
+a host-side exercise and is not committed.
+
+Anything beyond the CRC needs real hardware: build, flash, and watch the CSV
+stream. Unplugging a sensor is the quick way to exercise the error paths
+(`ERR_INIT` / `ERR_CRC`, LED pulse, auto re-init after 10 consecutive failures).
+
+## Notes
+
+`.pio/`, `.omo/`, `.junie/` and `.idea/` are gitignored tooling state, not
+project sources.
--- a/README.md
+++ b/README.md
@@ -0,0 +1,146 @@
+# Barometric pressure test
+
+## Goals
+
+1. Read barometric pressure from two sensors. One connected to the MCU with SPI and the other connected with I2C bus.
+2. The reading must be done in real time and data should be checked for validity.
+3. The output should be sent to a serial port.
+4. In case of reading error the LED should be turned on for 1 second. Each sensor has its own LED.
+
+## Hardware
+- MCU - STM32F042K6T6 (Nucleo-32, board MB1180) - 32 KB flash, 6 KB RAM
+- Barometric pressure sensor - 2x GY-63 (MS5611)
+
+## Wiring
+
+Pin assignments follow ST UM1956 Table 10 and the MB1180 C.2 schematic, and match
+the STM32duino defaults for this variant, so `Wire.begin()` and `SPI.begin()` need
+no explicit pin overrides.
+
+![Wiring diagram](docs/wiring.png)
+
+Vector source: [`docs/wiring.svg`](docs/wiring.svg). Pins there are grouped by
+function, not by physical header order.
+
+The same wiring drawn physically, with the real header order from UM1956
+Table 10, so it is clear which hole each jumper goes into:
+
+![Physical wiring diagram](docs/wiring-physical.png)
+
+Vector source: [`docs/wiring-physical.svg`](docs/wiring-physical.svg). Note that
+SPI is split across both connectors: MOSI, MISO and CS are on CN3, while SCK is
+CN4 pin 15.
+
+### Baro0 - I2C (GY-63 <=> STM32F042)
+| GY-63 | Nucleo | STM32 | Note |
+|-------|--------|-------|------|
+| VCC | 3V3 | - | |
+| GND | GND | - | |
+| PS | 3.3V | - | high selects I2C |
+| SDA | A4 | PB7 | `PIN_WIRE_SDA`, reaches the A4 pad through SB18 |
+| SCL | A5 | PB6 | `PIN_WIRE_SCL`, reaches the A5 pad through SB16 |
+| CSB | GND | - | sets address `0x77`; tie to VCC for `0x76` |
+
+The MS5611 address is `1110 11Cx`, where C is the **complement** of CSB
+(datasheet p.12). `CSB` must not be left floating. If you strap it high, change
+`BARO_I2C_ADDRESS` in [`include/config.h`](include/config.h).
+
+### Baro1 - SPI (GY-63 <=> STM32F042)
+| GY-63 | Nucleo | STM32 | Note |
+|-------|--------|-------|------|
+| VCC | 3V3 | - | |
+| GND | GND | - | |
+| PS | GND | - | low selects SPI |
+| SCLK | D13 | PB3 | SPI1_SCK |
+| SDI | D11 | PB5 | SPI1_MOSI |
+| SDO | D12 | PB4 | SPI1_MISO |
+| CSB | D10 | PA11 | chip select, `BARO_SPI_CS_PIN` |
+
+SPI1 has to stay on this pin group. The alternative group (PA5/PA6/PA7) is
+unusable while I2C is active: with the factory-default solder bridges SB16 and
+SB18 closed, PA6 shares a net with PB6 (SCL) and PA5 shares a net with PB7 (SDA).
+
+### Error LEDs
+
+Two external LEDs, one per sensor, each in series with roughly 510 R to GND.
+
+| Signal | Nucleo | STM32 |
+|--------|--------|-------|
+| I2C sensor error | D3 | PB0 |
+| SPI sensor error | D6 | PB1 |
+
+Both pins are set in [`include/config.h`](include/config.h).
+
+## Build and run
+
+```sh
+pio run              # build
+pio run -t upload    # flash over ST-LINK
+pio device monitor   # 115200 baud, ST-LINK Virtual COM Port
+```
+
+`Serial` is USART2 on PA2/PA15, wired to the ST-LINK VCP, so no USB-serial
+adapter is needed.
+
+The image is close to the flash ceiling (about 89% of 32 KB). `-flto` and
+`-fsingle-precision-constant` in [`platformio.ini`](platformio.ini) are required,
+not cosmetic: both MS5611 drivers write their compensation maths with unsuffixed
+double literals, which otherwise links roughly 5 KB of double soft-float helpers
+and overflows flash.
+
+## Output
+
+CSV at 10 Hz on the serial port, with a header line printed once at startup:
+
+```
+ms,i2c_status,i2c_p_mbar,i2c_t_c,spi_status,spi_p_mbar,spi_t_c
+1043,OK,1013.24,24.31,OK,1013.19,24.44
+1143,OK,1013.25,24.31,ERR_READ,1013.19,24.44
+```
+
+Pressure is in mbar, temperature in degrees C, both to two decimals. The status
+column is authoritative: when it is not `OK`, the two values next to it are the
+last ones successfully computed, not fresh readings.
+
+| Status | Meaning |
+|--------|---------|
+| `INIT` | not initialised yet |
+| `OK` | reading passed every check |
+| `ERR_INIT` | reset / PROM handshake did not complete |
+| `ERR_CRC` | factory calibration failed CRC-4, or is all-zero / all-ones |
+| `ERR_READ` | driver reported a bus or ADC failure |
+| `ERR_RANGE` | value outside the MS5611 operating envelope |
+| `ERR_STALE` | sensor still answers but stopped producing new samples |
+
+Sensors are polled continuously, as fast as the ADC conversions allow (roughly
+90 Hz per sensor at OSR 1024); the serial report is throttled to 10 Hz.
+
+## Validation
+
+Neither driver library detects every failure on its own, so the checks live in
+[`BaroChannel`](src/baro_channel.cpp):
+
+- **PROM CRC-4** at init, per datasheet p.13 / AN520. Neither library verifies it.
+- **All-zero / all-ones PROM screening**, because an absent part reads back as
+  one of those, `0xFFFF` is accepted by both libraries' `reset()`, and an
+  all-zero PROM satisfies the CRC.
+- **Driver return code** on every read.
+- **Range check** against 10..1200 mbar and -40..+85 C. This is the one that
+  catches the failure the I2C driver misses entirely: an ADC read taken before
+  the conversion completes returns 0 (datasheet p.11), and `MS5611::read()`
+  still reports `MS5611_READ_OK` for it.
+- **Stale detection** - bit-identical pressure *and* temperature for 32
+  consecutive polls means the part stopped converting while still acknowledging.
+
+Any failure lights that sensor's LED for 1 second (non-blocking, extended if
+further errors follow). After 10 consecutive failures the channel is taken back
+through the full reset and PROM handshake, so a sensor that was unplugged and
+reconnected recovers on its own.
+
+The CRC-4 implementation was verified differentially on the host against the
+independent NuttX reference over 200000 randomised PROM images.
+
+## References
+- [Barometric Pressure Sensor MS5611](docs/datasheets/ENG_DS_MS5611-01BA03_B3.pdf)
+- [STM32F042K6T6 Nucleo](docs/datasheets/DS_stm32f042k6.pdf)
+- [STM32 Nucleo-32 boards (MB1180)](https://www.st.com/resource/en/user_manual/um1956-stm32-nucleo32-boards-mb1180-stmicroelectronics.pdf) - UM1956, LED and connector tables
--- a/docs/datasheets/DB_nucleo-f042k6.pdf
+++ b/docs/datasheets/DB_nucleo-f042k6.pdf
--- a/docs/datasheets/DS_stm32f042k6.pdf
+++ b/docs/datasheets/DS_stm32f042k6.pdf
--- a/docs/datasheets/ENG_DS_MS5611-01BA03_B3.pdf
+++ b/docs/datasheets/ENG_DS_MS5611-01BA03_B3.pdf
--- a/docs/wiring-physical.png
+++ b/docs/wiring-physical.png
--- a/docs/wiring-physical.svg
+++ b/docs/wiring-physical.svg
@@ -0,0 +1,330 @@
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+  <rect x="0" y="0" width="1600" height="1120" fill="#ffffff"/>
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+  <text class="title" x="800" y="38" text-anchor="middle">Physical wiring  -  NUCLEO-F042K6  +  2 x GY-63 (MS5611)</text>
+  <text class="sub" x="800" y="62" text-anchor="middle">Board seen from the component side, USB at the top. Header order follows ST UM1956 Table 10. Highlighted pads are the ones this firmware uses.</text>
+
+  <!-- =================================================================== -->
+  <!--  NUCLEO-F042K6                                                       -->
+  <!-- =================================================================== -->
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+    <text class="bsilk" x="876" y="215" text-anchor="middle" font-size="9">CN4</text>
+  </g>
+
+  <!-- header pads: generated positions, CN3 x=714 / CN4 x=876, pin i y = 240+(i-1)*28 -->
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+    <!-- CN3 gold rings -->
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+    <text x="861" y="244" text-anchor="end">VIN</text> <text x="861" y="272" text-anchor="end">GND</text>
+    <text x="861" y="300" text-anchor="end">RST</text> <text x="861" y="328" text-anchor="end">5V</text>
+    <text x="861" y="356" text-anchor="end">A7</text>  <text x="861" y="384" text-anchor="end">A6</text>
+    <text x="861" y="412" text-anchor="end">A5</text>  <text x="861" y="440" text-anchor="end">A4</text>
+    <text x="861" y="468" text-anchor="end">A3</text>  <text x="861" y="496" text-anchor="end">A2</text>
+    <text x="861" y="524" text-anchor="end">A1</text>  <text x="861" y="552" text-anchor="end">A0</text>
+    <text x="861" y="580" text-anchor="end">REF</text> <text x="861" y="608" text-anchor="end">3V3</text>
+    <text x="861" y="636" text-anchor="end">D13</text>
+  </g>
+
+  <!-- =================================================================== -->
+  <!--  GY-63 #2  -  SPI,  left                                            -->
+  <!-- =================================================================== -->
+  <g id="gy63-spi" transform="translate(300,470)">
+    <rect x="0" y="0" width="150" height="190" rx="5" fill="#174a91" stroke="#0e2f5e" stroke-width="1.5"/>
+    <rect x="0" y="0" width="150" height="190" rx="5" fill="url(#sheen)"/>
+    <circle cx="26" cy="27" r="14" fill="#c9a227"/><circle cx="26" cy="27" r="10" fill="#ffffff"/>
+    <g fill="#d9b64a">
+      <circle cx="133" cy="24" r="8"/><circle cx="133" cy="47" r="8"/><circle cx="133" cy="70" r="8"/>
+      <circle cx="133" cy="93" r="8"/><circle cx="133" cy="116" r="8"/><circle cx="133" cy="139" r="8"/>
+      <circle cx="133" cy="162" r="8"/>
+    </g>
+    <g fill="#10305f">
+      <circle cx="133" cy="24" r="3.6"/><circle cx="133" cy="47" r="3.6"/><circle cx="133" cy="70" r="3.6"/>
+      <circle cx="133" cy="93" r="3.6"/><circle cx="133" cy="116" r="3.6"/><circle cx="133" cy="139" r="3.6"/>
+      <circle cx="133" cy="162" r="3.6"/>
+    </g>
+    <text class="silk" x="120" y="28"  text-anchor="end">PS</text>
+    <text class="silk" x="120" y="51"  text-anchor="end">SDO</text>
+    <text class="silk" x="120" y="74"  text-anchor="end">CSB</text>
+    <text class="silk" x="120" y="97"  text-anchor="end">SDA</text>
+    <text class="silk" x="120" y="120" text-anchor="end">SCL</text>
+    <text class="silk" x="120" y="143" text-anchor="end">GND</text>
+    <text class="silk" x="120" y="166" text-anchor="end">VCC</text>
+    <rect x="36" y="88" width="52" height="42" rx="3" fill="#b9bfc6" stroke="#7e878f" stroke-width="1.2"/>
+    <rect x="40" y="92" width="44" height="34" rx="2" fill="#a8aeb5"/>
+    <circle cx="62" cy="109" r="7" fill="#5c646c"/><circle cx="62" cy="109" r="4" fill="#3a4148"/>
+    <rect x="38" y="156" width="20" height="11" rx="1" fill="#1b1b1b"/>
+    <rect x="66" y="156" width="20" height="11" rx="1" fill="#1b1b1b"/>
+    <rect x="72" y="140" width="14" height="9"  rx="1" fill="#d8801f"/>
+    <rect x="36" y="70" width="18" height="10" rx="1" fill="#1b1b1b"/>
+    <rect x="68" y="70" width="18" height="10" rx="1" fill="#1b1b1b"/>
+    <rect x="56" y="52" width="12" height="8"  rx="1" fill="#1b1b1b"/>
+    <rect x="74" y="52" width="12" height="8"  rx="1" fill="#1b1b1b"/>
+    <text class="silkb" x="14" y="150" transform="rotate(-90,14,150)">MS5611</text>
+  </g>
+  <text class="hdr" x="375" y="452" text-anchor="middle">GY-63 #2  -  SPI</text>
+
+  <!-- =================================================================== -->
+  <!--  GY-63 #1  -  I2C,  right                                           -->
+  <!-- =================================================================== -->
+  <g id="gy63-i2c" transform="translate(1150,330)">
+    <rect x="0" y="0" width="150" height="190" rx="5" fill="#174a91" stroke="#0e2f5e" stroke-width="1.5"/>
+    <rect x="0" y="0" width="150" height="190" rx="5" fill="url(#sheen)"/>
+    <circle cx="124" cy="163" r="14" fill="#c9a227"/><circle cx="124" cy="163" r="10" fill="#ffffff"/>
+    <g fill="#d9b64a">
+      <circle cx="17" cy="24" r="8"/><circle cx="17" cy="47" r="8"/><circle cx="17" cy="70" r="8"/>
+      <circle cx="17" cy="93" r="8"/><circle cx="17" cy="116" r="8"/><circle cx="17" cy="139" r="8"/>
+      <circle cx="17" cy="162" r="8"/>
+    </g>
+    <g fill="#10305f">
+      <circle cx="17" cy="24" r="3.6"/><circle cx="17" cy="47" r="3.6"/><circle cx="17" cy="70" r="3.6"/>
+      <circle cx="17" cy="93" r="3.6"/><circle cx="17" cy="116" r="3.6"/><circle cx="17" cy="139" r="3.6"/>
+      <circle cx="17" cy="162" r="3.6"/>
+    </g>
+    <text class="silk" x="30" y="28">VCC</text><text class="silk" x="30" y="51">GND</text>
+    <text class="silk" x="30" y="74">SCL</text><text class="silk" x="30" y="97">SDA</text>
+    <text class="silk" x="30" y="120">CSB</text><text class="silk" x="30" y="143">SDO</text>
+    <text class="silk" x="30" y="166">PS</text>
+    <rect x="62" y="58" width="52" height="42" rx="3" fill="#b9bfc6" stroke="#7e878f" stroke-width="1.2"/>
+    <rect x="66" y="62" width="44" height="34" rx="2" fill="#a8aeb5"/>
+    <circle cx="88" cy="79" r="7" fill="#5c646c"/><circle cx="88" cy="79" r="4" fill="#3a4148"/>
+    <rect x="64" y="22" width="20" height="11" rx="1" fill="#1b1b1b"/>
+    <rect x="92" y="22" width="20" height="11" rx="1" fill="#1b1b1b"/>
+    <rect x="64" y="40" width="14" height="9" rx="1" fill="#d8801f"/>
+    <rect x="64" y="110" width="18" height="10" rx="1" fill="#1b1b1b"/>
+    <rect x="96" y="110" width="18" height="10" rx="1" fill="#1b1b1b"/>
+    <rect x="64" y="128" width="12" height="8" rx="1" fill="#1b1b1b"/>
+    <rect x="82" y="128" width="12" height="8" rx="1" fill="#1b1b1b"/>
+    <text class="silkb" x="140" y="38" transform="rotate(90,140,38)">MS5611</text>
+  </g>
+  <text class="hdr" x="1225" y="312" text-anchor="middle">GY-63 #1  -  I2C</text>
+
+  <!-- =================================================================== -->
+  <!--  POWER RAILS                                                        -->
+  <!-- =================================================================== -->
+  <path class="rail-g" d="M 300,940 L 1290,940"/>
+  <path class="rail-v" d="M 240,988 L 1400,988"/>
+  <text class="mono" x="1302" y="945" font-size="13" fill="#22252a">GND rail</text>
+  <text class="mono" x="1412" y="993" font-size="13" fill="#d62828">3V3 rail</text>
+
+  <!-- =================================================================== -->
+  <!--  JUMPER WIRES                                                       -->
+  <!-- =================================================================== -->
+  <!-- I2C signals: module #1 -> CN4 A5 / A4 -->
+  <path class="jw c-scl" d="M 1167,400 C 1080,398 970,406 883,408"/>
+  <path class="jw c-sda" d="M 1167,423 C 1080,422 970,432 883,436"/>
+
+  <!-- SPI signals: module #2 -> CN3 D10 / D11 / D12 -->
+  <path class="jw c-cs"   d="M 433,540 C 540,545 610,570 707,576"/>
+  <path class="jw c-mosi" d="M 433,563 C 540,570 610,598 707,604"/>
+  <path class="jw c-miso" d="M 433,517 C 560,520 620,614 707,632"/>
+  <!-- SCK has to travel to the far side: CN4 pin 15 -->
+  <path class="jw c-sck"  d="M 433,586 C 560,600 520,732 660,742 C 820,752 962,730 962,670 C 962,642 918,632 883,632"/>
+
+  <!-- power taps from the board -->
+  <path class="jw c-v33" d="M 883,604 C 940,616 946,860 1006,988"/>
+  <path class="jw c-gnd" d="M 883,268 C 1006,300 1028,760 1046,940"/>
+
+  <!-- module #1 power and straps -->
+  <path class="jw c-v33" d="M 1167,354 C 1062,404 1010,790 1120,988"/>
+  <path class="jw c-gnd" d="M 1167,377 C 1090,448 1064,824 1176,940"/>
+  <path class="jw c-gnd" d="M 1167,446 C 1122,600 1152,852 1244,940"/>
+  <path class="jw c-v33" d="M 1167,492 C 1206,650 1258,880 1334,988"/>
+
+  <!-- module #2 power and strap -->
+  <path class="jw c-v33" d="M 433,632 C 540,730 520,880 500,988"/>
+  <path class="jw c-gnd" d="M 433,609 C 510,700 410,860 380,940"/>
+  <path class="jw c-gnd" d="M 433,494 C 540,506 560,812 440,940"/>
+
+  <!-- LED drives -->
+  <path class="jw c-led" d="M 707,380 C 600,420 500,652 560,772"/>
+  <path class="jw c-led" d="M 707,464 C 648,512 662,664 700,772"/>
+
+  <!-- =================================================================== -->
+  <!--  LEDS                                                               -->
+  <!-- =================================================================== -->
+  <g id="led1">
+    <rect x="545" y="772" width="30" height="48" rx="2" fill="#ffffff" stroke="#22252a" stroke-width="2"/>
+    <line x1="560" y1="820" x2="560" y2="842" stroke="#2a9d4a" stroke-width="4.6" stroke-linecap="round"/>
+    <polygon points="542,842 578,842 560,872" fill="#ffffff" stroke="#22252a" stroke-width="2"/>
+    <line x1="540" y1="872" x2="580" y2="872" stroke="#22252a" stroke-width="2.6" stroke-linecap="round"/>
+    <path class="jw c-gnd" d="M 560,872 L 560,940"/>
+    <text class="mono" x="590" y="800" font-size="12">510 R</text>
+    <text x="590" y="862" font-size="12">I2C error</text>
+  </g>
+  <g id="led2">
+    <rect x="685" y="772" width="30" height="48" rx="2" fill="#ffffff" stroke="#22252a" stroke-width="2"/>
+    <line x1="700" y1="820" x2="700" y2="842" stroke="#2a9d4a" stroke-width="4.6" stroke-linecap="round"/>
+    <polygon points="682,842 718,842 700,872" fill="#ffffff" stroke="#22252a" stroke-width="2"/>
+    <line x1="680" y1="872" x2="720" y2="872" stroke="#22252a" stroke-width="2.6" stroke-linecap="round"/>
+    <path class="jw c-gnd" d="M 700,872 L 700,940"/>
+    <text class="mono" x="734" y="800" font-size="12">510 R</text>
+    <text x="734" y="862" font-size="12">SPI error</text>
+  </g>
+
+  <!-- junction dots where a wire really meets a rail -->
+  <g fill="#22252a">
+    <circle cx="1046" cy="940" r="5"/><circle cx="1176" cy="940" r="5"/><circle cx="1244" cy="940" r="5"/>
+    <circle cx="380"  cy="940" r="5"/><circle cx="440"  cy="940" r="5"/>
+    <circle cx="560"  cy="940" r="5"/><circle cx="700"  cy="940" r="5"/>
+  </g>
+  <g fill="#d62828">
+    <circle cx="1006" cy="988" r="5"/><circle cx="1120" cy="988" r="5"/>
+    <circle cx="1334" cy="988" r="5"/><circle cx="500"  cy="988" r="5"/>
+  </g>
+
+  <!-- =================================================================== -->
+  <!--  LEGEND + NOTES                                                     -->
+  <!-- =================================================================== -->
+  <rect class="panel" x="60" y="742" width="300" height="176" rx="6"/>
+  <text class="hdr" x="76" y="768">WIRE COLOURS</text>
+  <g stroke-width="4.6" stroke-linecap="round">
+    <line x1="80" y1="790" x2="118" y2="790" stroke="#d62828"/><text x="128" y="795" font-size="12.5">3V3</text>
+    <line x1="80" y1="812" x2="118" y2="812" stroke="#22252a"/><text x="128" y="817" font-size="12.5">GND</text>
+    <line x1="80" y1="834" x2="118" y2="834" stroke="#00a0b0"/><text x="128" y="839" font-size="12.5">SCL</text>
+    <line x1="80" y1="856" x2="118" y2="856" stroke="#1f6feb"/><text x="128" y="861" font-size="12.5">SDA</text>
+    <line x1="80" y1="878" x2="118" y2="878" stroke="#2a9d4a"/><text x="128" y="883" font-size="12.5">LED drive</text>
+    <line x1="212" y1="790" x2="250" y2="790" stroke="#e07b00"/><text x="260" y="795" font-size="12.5">SCK</text>
+    <line x1="212" y1="812" x2="250" y2="812" stroke="#b5179e"/><text x="260" y="817" font-size="12.5">MOSI</text>
+    <line x1="212" y1="834" x2="250" y2="834" stroke="#7209b7"/><text x="260" y="839" font-size="12.5">MISO</text>
+    <line x1="212" y1="856" x2="250" y2="856" stroke="#c99700"/><text x="260" y="861" font-size="12.5">CS</text>
+  </g>
+
+  <rect class="panel" x="60" y="92" width="490" height="336" rx="6"/>
+  <text class="hdr" x="78" y="118">NOTES</text>
+  <text class="note" x="78" y="144">SPI is split across both headers: MOSI / MISO / CS sit on CN3</text>
+  <text class="note" x="78" y="162">(D11 / D12 / D10) but SCK is CN4 pin 15 (D13). One jumper has</text>
+  <text class="note" x="78" y="180">to cross to the far side of the board - that is normal here.</text>
+  <text class="note" x="78" y="206">I2C is wired to A5 / A4 on CN4. Solder bridges SB16 / SB18 are</text>
+  <text class="note" x="78" y="224">closed from the factory, which puts PB6 / PB7 on those pads.</text>
+  <text class="note" x="78" y="242">D5 / D4 on CN3 are the same two nets - either pair works.</text>
+  <text class="note" x="78" y="268">Do not power the sensors from 5V. MS5611 runs on 1.8 - 3.6 V.</text>
+  <text class="note" x="78" y="294">Both LEDs are external and active high: the GPIO drives the</text>
+  <text class="note" x="78" y="312">anode through 510 R. LD3 on the board cannot be used - it</text>
+  <text class="note" x="78" y="330">shares PB3 with SPI1 SCK.</text>
+  <text class="note" x="78" y="356">A dot means the wire connects to the rail. Wires that merely</text>
+  <text class="note" x="78" y="374">cross without a dot are not connected.</text>
+  <text class="note" x="78" y="400">Sensor #2 is drawn turned 180 deg so its pad row faces the board -</text>
+  <text class="note" x="78" y="418">that is why its pads read PS first and VCC last.</text>
+</svg>
--- a/docs/wiring.png
+++ b/docs/wiring.png
--- a/docs/wiring.svg
+++ b/docs/wiring.svg
@@ -0,0 +1,226 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1500 1010" width="1500" height="1010">
+  <style>
+    .bg    { fill: #ffffff; }
+    text   { font-family: "Helvetica Neue", Arial, sans-serif; fill: #1b2733; }
+    .mono  { font-family: "SF Mono", "DejaVu Sans Mono", Menlo, Consolas, monospace; }
+    .title { font-size: 25px; font-weight: 700; }
+    .sub   { font-size: 13.5px; fill: #5a6b7d; }
+    .bname { font-size: 19px; font-weight: 700; }
+    .mname { font-size: 16px; font-weight: 700; }
+    .spec  { font-size: 12.5px; fill: #5a6b7d; }
+    .pin   { font-size: 13px; }
+    .net   { font-size: 12px; }
+    .note  { font-size: 12.5px; }
+    .lgd   { font-size: 13px; }
+
+    .board { fill: #e8eef6; stroke: #2f4f7f; stroke-width: 2; }
+    .modu  { fill: #faf3e6; stroke: #96703a; stroke-width: 2; }
+    .panel { fill: #f4f6f8; stroke: #b6c1cc; stroke-width: 1.5; }
+
+    .w        { fill: none; stroke-width: 2.4; stroke-linecap: round; stroke-linejoin: round; }
+    .w-v33    { stroke: #cc2b2b; }
+    .w-gnd    { stroke: #2b2f36; }
+    .w-i2c    { stroke: #1668c4; }
+    .w-spi    { stroke: #d98207; }
+    .w-led    { stroke: #1f9350; }
+    .rail     { fill: none; stroke: #cc2b2b; stroke-width: 3.4; stroke-linecap: round; }
+
+    .stub  { stroke: #2f4f7f; stroke-width: 2; }
+    .pad   { fill: #96703a; }
+    .jn    { fill: #cc2b2b; }
+    .part  { fill: #ffffff; stroke: #2b2f36; stroke-width: 2; }
+    .gsym  { fill: none; stroke: #2b2f36; stroke-width: 2; stroke-linecap: round; }
+  </style>
+
+  <rect class="bg" x="0" y="0" width="1500" height="1010"/>
+
+  <text class="title" x="750" y="38" text-anchor="middle">NUCLEO-F042K6  +  2 x GY-63 (MS5611)  -  wiring</text>
+  <text class="sub" x="750" y="60" text-anchor="middle">Logical net diagram. Pins are grouped by function, not by physical header order - see UM1956 Table 10 for the real connector layout.</text>
+
+  <!-- ============================ POWER SOURCE ============================ -->
+  <rect class="panel" x="90" y="90" width="300" height="86" rx="6"/>
+  <text class="mname" x="106" y="114">POWER SOURCE</text>
+  <text class="spec" x="106" y="136">USB Micro-B to CN1, 5 V from the host PC</text>
+  <text class="spec" x="106" y="154">ST-LINK V2-1 + on-board 3.3 V LDO</text>
+  <text class="spec" x="106" y="170">same cable carries the serial VCP</text>
+
+  <!-- USB feed into the board, hopping over the 3V3 rail -->
+  <path class="w w-gnd" d="M 390,133 L 630,133 L 630,196 A 9,9 0 0,0 630,214 L 630,250"/>
+  <text class="net mono" x="452" y="126">5 V + serial</text>
+
+  <!-- ============================ 3V3 RAIL ============================ -->
+  <path class="rail" d="M 490,205 L 1350,205"/>
+  <text class="pin mono" x="1362" y="210" fill="#cc2b2b">+3V3</text>
+  <circle class="jn" cx="490" cy="205" r="4.5"/>
+  <circle class="jn" cx="720" cy="205" r="4.5"/>
+  <circle class="jn" cx="1050" cy="205" r="4.5"/>
+
+  <!-- ============================ NUCLEO BOARD ============================ -->
+  <rect class="board" x="600" y="250" width="300" height="540" rx="10"/>
+  <text class="bname" x="750" y="332" text-anchor="middle">NUCLEO-F042K6</text>
+  <text class="spec" x="750" y="354" text-anchor="middle">STM32F042K6T6  -  board MB1180</text>
+  <text class="spec" x="750" y="372" text-anchor="middle">32 KB flash  /  6 KB RAM</text>
+
+  <!-- top edge pins -->
+  <line class="stub" x1="630" y1="250" x2="630" y2="240"/>
+  <line class="stub" x1="720" y1="250" x2="720" y2="240"/>
+  <line class="stub" x1="790" y1="250" x2="790" y2="240"/>
+  <text class="pin mono" x="630" y="272" text-anchor="middle">CN1</text>
+  <text class="pin mono" x="720" y="272" text-anchor="middle">3V3</text>
+  <text class="pin mono" x="790" y="272" text-anchor="middle">GND</text>
+
+  <!-- left edge pins: I2C -->
+  <line class="stub" x1="600" y1="440" x2="590" y2="440"/>
+  <line class="stub" x1="600" y1="475" x2="590" y2="475"/>
+  <text class="pin mono" x="616" y="444">PB6 / D5 <tspan class="spec">(pad A5)</tspan></text>
+  <text class="pin mono" x="616" y="479">PB7 / D4 <tspan class="spec">(pad A4)</tspan></text>
+
+  <!-- right edge pins: SPI -->
+  <line class="stub" x1="900" y1="440" x2="910" y2="440"/>
+  <line class="stub" x1="900" y1="475" x2="910" y2="475"/>
+  <line class="stub" x1="900" y1="510" x2="910" y2="510"/>
+  <line class="stub" x1="900" y1="545" x2="910" y2="545"/>
+  <text class="pin mono" x="884" y="444" text-anchor="end">PB3 / D13</text>
+  <text class="pin mono" x="884" y="479" text-anchor="end">PB5 / D11</text>
+  <text class="pin mono" x="884" y="514" text-anchor="end">PA11 / D10</text>
+  <text class="pin mono" x="884" y="549" text-anchor="end">PB4 / D12</text>
+
+  <!-- bottom edge pins: LEDs -->
+  <line class="stub" x1="660" y1="790" x2="660" y2="800"/>
+  <line class="stub" x1="850" y1="790" x2="850" y2="800"/>
+  <text class="pin mono" x="660" y="778" text-anchor="middle">PB0 / D3</text>
+  <text class="pin mono" x="850" y="778" text-anchor="middle">PB1 / D6</text>
+
+  <text class="spec" x="750" y="702" text-anchor="middle">USART2 = PA2 / PA15 to ST-LINK VCP</text>
+  <text class="spec" x="750" y="720" text-anchor="middle">115200 8N1  -  no USB-serial adapter needed</text>
+
+  <!-- ============================ GY-63 #1 : I2C ============================ -->
+  <rect class="modu" x="110" y="280" width="280" height="340" rx="8"/>
+  <text class="mname" x="250" y="312" text-anchor="middle">GY-63  #1</text>
+  <text class="spec" x="250" y="334" text-anchor="middle">MS5611-01BA03  -  I2C mode</text>
+
+  <circle class="pad" cx="390" cy="370" r="4"/>
+  <circle class="pad" cx="390" cy="405" r="4"/>
+  <circle class="pad" cx="390" cy="440" r="4"/>
+  <circle class="pad" cx="390" cy="475" r="4"/>
+  <circle class="pad" cx="390" cy="510" r="4"/>
+  <circle class="pad" cx="390" cy="545" r="4"/>
+  <circle class="pad" cx="390" cy="580" r="4"/>
+  <text class="pin mono" x="376" y="374" text-anchor="end">VCC</text>
+  <text class="pin mono" x="376" y="409" text-anchor="end">GND</text>
+  <text class="pin mono" x="376" y="444" text-anchor="end">SCL</text>
+  <text class="pin mono" x="376" y="479" text-anchor="end">SDA</text>
+  <text class="pin mono" x="376" y="514" text-anchor="end">CSB</text>
+  <text class="pin mono" x="376" y="549" text-anchor="end">SDO</text>
+  <text class="pin mono" x="376" y="584" text-anchor="end">PS</text>
+  <text class="spec" x="284" y="549" text-anchor="end">not connected</text>
+
+  <!-- ============================ GY-63 #2 : SPI ============================ -->
+  <rect class="modu" x="1110" y="280" width="280" height="340" rx="8"/>
+  <text class="mname" x="1250" y="312" text-anchor="middle">GY-63  #2</text>
+  <text class="spec" x="1250" y="334" text-anchor="middle">MS5611-01BA03  -  SPI mode</text>
+
+  <circle class="pad" cx="1110" cy="370" r="4"/>
+  <circle class="pad" cx="1110" cy="405" r="4"/>
+  <circle class="pad" cx="1110" cy="440" r="4"/>
+  <circle class="pad" cx="1110" cy="475" r="4"/>
+  <circle class="pad" cx="1110" cy="510" r="4"/>
+  <circle class="pad" cx="1110" cy="545" r="4"/>
+  <circle class="pad" cx="1110" cy="580" r="4"/>
+  <text class="pin mono" x="1124" y="374">VCC</text>
+  <text class="pin mono" x="1124" y="409">GND</text>
+  <text class="pin mono" x="1124" y="444">SCL  <tspan class="spec">= SCLK</tspan></text>
+  <text class="pin mono" x="1124" y="479">SDA  <tspan class="spec">= SDI / MOSI</tspan></text>
+  <text class="pin mono" x="1124" y="514">CSB  <tspan class="spec">= CS</tspan></text>
+  <text class="pin mono" x="1124" y="549">SDO  <tspan class="spec">= MISO</tspan></text>
+  <text class="pin mono" x="1124" y="584">PS</text>
+
+  <!-- ============================ POWER WIRING ============================ -->
+  <path class="w w-v33" d="M 390,370 L 490,370 L 490,205"/>
+  <path class="w w-v33" d="M 1110,370 L 1050,370 L 1050,205"/>
+  <path class="w w-v33" d="M 720,250 L 720,205"/>
+
+  <path class="w w-gnd" d="M 390,405 L 530,405 L 530,232"/>
+  <path class="w w-gnd" d="M 1110,405 L 1010,405 L 1010,232"/>
+  <path class="w w-gnd" d="M 790,250 L 790,232"/>
+  <g class="gsym"><line x1="516" y1="232" x2="544" y2="232"/><line x1="521" y1="238" x2="539" y2="238"/><line x1="526" y1="244" x2="534" y2="244"/></g>
+  <g class="gsym"><line x1="996" y1="232" x2="1024" y2="232"/><line x1="1001" y1="238" x2="1019" y2="238"/><line x1="1006" y1="244" x2="1014" y2="244"/></g>
+  <g class="gsym"><line x1="776" y1="232" x2="804" y2="232"/><line x1="781" y1="238" x2="799" y2="238"/><line x1="786" y1="244" x2="794" y2="244"/></g>
+
+  <!-- ============================ I2C SIGNALS ============================ -->
+  <path class="w w-i2c" d="M 390,440 L 600,440"/>
+  <path class="w w-i2c" d="M 390,475 L 600,475"/>
+  <text class="net mono" x="495" y="432" text-anchor="middle" fill="#1668c4">SCL</text>
+  <text class="net mono" x="495" y="467" text-anchor="middle" fill="#1668c4">SDA</text>
+
+  <!-- CSB -> GND  (address 0x77) -->
+  <path class="w w-gnd" d="M 390,510 L 460,510 L 460,548"/>
+  <g class="gsym"><line x1="446" y1="548" x2="474" y2="548"/><line x1="451" y1="554" x2="469" y2="554"/><line x1="456" y1="560" x2="464" y2="560"/></g>
+  <text class="net" x="484" y="524" fill="#5a6b7d">0x77</text>
+
+  <!-- PS -> 3V3  (selects I2C) -->
+  <path class="w w-v33" d="M 390,580 L 545,580 L 545,556"/>
+  <line class="w w-v33" x1="531" y1="556" x2="559" y2="556"/>
+  <text class="net mono" x="545" y="546" text-anchor="middle" fill="#cc2b2b">3V3</text>
+
+  <!-- ============================ SPI SIGNALS ============================ -->
+  <path class="w w-spi" d="M 900,440 L 1110,440"/>
+  <path class="w w-spi" d="M 900,475 L 1110,475"/>
+  <path class="w w-spi" d="M 900,510 L 1110,510"/>
+  <path class="w w-spi" d="M 900,545 L 1110,545"/>
+  <text class="net mono" x="1005" y="432" text-anchor="middle" fill="#d98207">SCK</text>
+  <text class="net mono" x="1005" y="467" text-anchor="middle" fill="#d98207">MOSI</text>
+  <text class="net mono" x="1005" y="502" text-anchor="middle" fill="#d98207">CS</text>
+  <text class="net mono" x="1005" y="537" text-anchor="middle" fill="#d98207">MISO</text>
+
+  <!-- PS -> GND  (selects SPI) -->
+  <path class="w w-gnd" d="M 1110,580 L 1040,580 L 1040,614"/>
+  <g class="gsym"><line x1="1026" y1="614" x2="1054" y2="614"/><line x1="1031" y1="620" x2="1049" y2="620"/><line x1="1036" y1="626" x2="1044" y2="626"/></g>
+
+  <!-- ============================ ERROR LEDS ============================ -->
+  <!-- branch 1 : PB0 -->
+  <path class="w w-led" d="M 660,800 L 660,832"/>
+  <rect class="part" x="645" y="832" width="30" height="50" rx="2"/>
+  <path class="w w-led" d="M 660,882 L 660,906"/>
+  <polygon class="part" points="642,906 678,906 660,936"/>
+  <line class="w w-led" x1="640" y1="936" x2="680" y2="936"/>
+  <path class="w w-led" d="M 660,936 L 660,964"/>
+  <g class="gsym"><line x1="646" y1="964" x2="674" y2="964"/><line x1="651" y1="970" x2="669" y2="970"/><line x1="656" y1="976" x2="664" y2="976"/></g>
+  <text class="pin mono" x="690" y="862">510 R</text>
+  <text class="pin" x="690" y="926">LED  -  I2C sensor error</text>
+
+  <!-- branch 2 : PB1 -->
+  <path class="w w-led" d="M 850,800 L 850,832"/>
+  <rect class="part" x="835" y="832" width="30" height="50" rx="2"/>
+  <path class="w w-led" d="M 850,882 L 850,906"/>
+  <polygon class="part" points="832,906 868,906 850,936"/>
+  <line class="w w-led" x1="830" y1="936" x2="870" y2="936"/>
+  <path class="w w-led" d="M 850,936 L 850,964"/>
+  <g class="gsym"><line x1="836" y1="964" x2="864" y2="964"/><line x1="841" y1="970" x2="859" y2="970"/><line x1="846" y1="976" x2="854" y2="976"/></g>
+  <text class="pin mono" x="880" y="862">510 R</text>
+  <text class="pin" x="880" y="926">LED  -  SPI sensor error</text>
+
+  <!-- ============================ LEGEND ============================ -->
+  <rect class="panel" x="90" y="660" width="290" height="196" rx="6"/>
+  <text class="mname" x="106" y="686">LEGEND</text>
+  <line class="w w-v33" x1="110" y1="712" x2="150" y2="712"/><text class="lgd" x="162" y="717">3.3 V</text>
+  <line class="w w-gnd" x1="110" y1="742" x2="150" y2="742"/><text class="lgd" x="162" y="747">GND</text>
+  <line class="w w-i2c" x1="110" y1="772" x2="150" y2="772"/><text class="lgd" x="162" y="777">I2C bus</text>
+  <line class="w w-spi" x1="110" y1="802" x2="150" y2="802"/><text class="lgd" x="162" y="807">SPI bus</text>
+  <line class="w w-led" x1="110" y1="832" x2="150" y2="832"/><text class="lgd" x="162" y="837">LED drive (active high)</text>
+
+  <!-- ============================ NOTES ============================ -->
+  <rect class="panel" x="1080" y="660" width="330" height="284" rx="6"/>
+  <text class="mname" x="1096" y="686">NOTES</text>
+  <text class="note" x="1096" y="712">MS5611 is 3.3 V only (1.8 - 3.6 V).</text>
+  <text class="note" x="1096" y="730">Never feed it 5 V.</text>
+  <text class="note" x="1096" y="756">GY-63 boards normally carry SDA / SCL</text>
+  <text class="note" x="1096" y="774">pull-ups - check yours before adding any.</text>
+  <text class="note" x="1096" y="800">PS high selects I2C, PS low selects SPI.</text>
+  <text class="note" x="1096" y="826">CSB low on module #1 sets address 0x77</text>
+  <text class="note" x="1096" y="844">(BARO_I2C_ADDRESS in include/config.h).</text>
+  <text class="note" x="1096" y="870">SPI1 must stay on PB3 / PB4 / PB5: bridges</text>
+  <text class="note" x="1096" y="888">SB16 / SB18 tie PA5 / PA6 to the I2C net.</text>
+  <text class="note" x="1096" y="914">The Nucleo-32 has a single 3V3 pin -</text>
+  <text class="note" x="1096" y="932">distribute it over a breadboard rail.</text>
+</svg>
--- a/include/README
+++ b/include/README
@@ -0,0 +1,39 @@
+
+This directory is intended for project header files.
+
+A header file is a file containing C declarations and macro definitions
+to be shared between several project source files. You request the use of a
+header file in your project source file (C, C++, etc) located in `src` folder
+by including it, with the C preprocessing directive `#include'.
+
+```src/main.c
+
+#include "header.h"
+
+int main (void)
+{
+ ...
+}
+```
+
+Including a header file produces the same results as copying the header file
+into each source file that needs it. Such copying would be time-consuming
+and error-prone. With a header file, the related declarations appear
+in only one place. If they need to be changed, they can be changed in one
+place, and programs that include the header file will automatically use the
+new version when next recompiled. The header file eliminates the labor of
+finding and changing all the copies as well as the risk that a failure to
+find one copy will result in inconsistencies within a program.
+
+In C, the usual convention is to give header files names that end with `.h'.
+It is most portable to use only letters, digits, dashes, and underscores in
+header file names, and at most one dot.
+
+Read more about using header files in official GCC documentation:
+
+* Include Syntax
+* Include Operation
+* Once-Only Headers
+* Computed Includes
+
+https://gcc.gnu.org/onlinedocs/cpp/Header-Files.html
--- a/include/baro_channel.h
+++ b/include/baro_channel.h
@@ -0,0 +1,100 @@
+#pragma once
+
+#include <Arduino.h>
+#include <math.h>
+
+#include "config.h"
+#include "ms5611_crc.h"
+
+//
+//  One validated MS5611 measurement channel.
+//
+//  The class owns everything that is identical for both sensors - reset and
+//  PROM/CRC handshake, per-poll validation, error accounting and the 1 second
+//  error LED pulse. Only the five driver calls at the bottom are virtual, and
+//  they are implemented once for the I2C part and once for the SPI part.
+//
+
+enum class BaroStatus : uint8_t
+{
+  NotInitialised = 0,
+  Ok,
+  InitFailed,     //  reset/handshake did not complete
+  PromCrcError,   //  factory calibration failed its CRC-4 or is implausible
+  ReadError,      //  the driver reported a bus/ADC failure
+  OutOfRange,     //  values outside the MS5611 operating envelope
+  Stale,          //  sensor answers but has stopped producing new samples
+};
+
+//  Short, log-friendly name for a status - never returns NULL.
+const char *baroStatusName(BaroStatus status);
+
+
+class BaroChannel
+{
+public:
+  BaroChannel(const char *name, uint8_t ledPin)
+    : _name(name), _ledPin(ledPin) {}
+
+  //  Configures the LED pin and makes the first initialisation attempt.
+  void begin(uint32_t now);
+
+  //  One acquisition cycle: read, validate, update status and LED.
+  //  Blocks for roughly 2x the ADC conversion time of the configured OSR.
+  void poll(uint32_t now);
+
+  //  Releases the error LED once its hold time has elapsed. Cheap, call often.
+  void updateLed(uint32_t now);
+
+  const char *name() const        { return _name; }
+  BaroStatus  status() const      { return _status; }
+  bool        isOk() const        { return _status == BaroStatus::Ok; }
+  float       pressure() const    { return _pressure; }     //  mbar
+  float       temperature() const { return _temperature; }  //  degrees C
+  uint32_t    errorCount() const  { return _errorCount; }
+
+protected:
+  //  Not deleted through this type - keeps the vtable free of a destructor
+  //  slot and avoids dragging in operator delete.
+  ~BaroChannel() = default;
+
+  //  Reset the part and apply the configured oversampling.
+  virtual bool  driverBegin() = 0;
+  //  Fill prom[0..7] with the factory calibration words.
+  virtual void  driverReadProm(uint16_t *prom) = 0;
+  //  True when the driver reported MS5611_READ_OK.
+  virtual bool  driverRead() = 0;
+  virtual float driverPressure() = 0;
+  virtual float driverTemperature() = 0;
+
+private:
+  bool tryInit(uint32_t now);
+  void fail(BaroStatus status, uint32_t now);
+
+  const char *_name;
+  uint8_t     _ledPin;
+
+  BaroStatus  _status          = BaroStatus::NotInitialised;
+  bool        _initialised     = false;
+  uint32_t    _lastInitAttempt = 0;
+
+  float       _pressure        = NAN;
+  float       _temperature     = NAN;
+
+  uint8_t     _repeatCount     = 0;
+  uint8_t     _errorStreak     = 0;
+  uint32_t    _errorCount      = 0;
+
+  bool        _ledOn           = false;
+  uint32_t    _ledOffAt        = 0;
+};
+
+
+//
+//  The two concrete channels are built in their own translation units on
+//  purpose: MS5611.h and MS5611_SPI.h each define `enum osr_t` and their own
+//  MS5611_READ_OK, so including both in one file does not compile. These
+//  accessors hand out the instances without leaking either driver header.
+//
+BaroChannel &baroI2cChannel();
+BaroChannel &baroSpiChannel();
--- a/include/config.h
+++ b/include/config.h
@@ -0,0 +1,120 @@
+#pragma once
+
+#include <Arduino.h>
+
+//
+//  Project-wide hardware and behaviour configuration.
+//
+//  Board : NUCLEO-F042K6  (STM32F042K6T6, LQFP32, board MB1180)
+//          32 KB flash / 6 KB RAM
+//  Sensor: 2x GY-63 breakout carrying a MS5611-01BA03
+//          one on I2C1, one on SPI1
+//
+//  Pin facts below are taken from ST UM1956 "STM32 Nucleo-32 boards (MB1180)"
+//  Table 10 + the MB1180 C.2 schematic, cross-checked against the STM32duino
+//  variant files for NUCLEO_F042K6.
+//
+
+// ---------------------------------------------------------------------------
+//  I2C sensor  ("baro0")
+// ---------------------------------------------------------------------------
+//  STM32duino default Wire pins for this variant:
+//      PIN_WIRE_SDA = PB7  (Arduino D4, routed to the A4 header pad via SB18)
+//      PIN_WIRE_SCL = PB6  (Arduino D5, routed to the A5 header pad via SB16)
+//  Both are the STM32duino defaults, so plain Wire.begin() selects them and no
+//  explicit setSDA()/setSCL() call is required.
+//
+//  MS5611 I2C address is 1110 11Cx where C is the COMPLEMENT of the CSB pin
+//  (datasheet p.12), therefore:
+//      CSB tied to GND -> 0x77
+//      CSB tied to VCC -> 0x76
+//  Change this if the GY-63 CSB pad is strapped high.
+#define BARO_I2C_ADDRESS        0x77
+
+// ---------------------------------------------------------------------------
+//  SPI sensor  ("baro1")
+// ---------------------------------------------------------------------------
+//  SPI1 MUST stay on the PB3/PB4/PB5 group on this board.
+//
+//  The alternative SPI1 group (PA5 SCK / PA6 MISO / PA7 MOSI) is NOT usable
+//  here: with the factory-default solder bridges SB16 and SB18 closed, PA6 is
+//  tied to the same net as PB6 (I2C SCL) and PA5 is tied to the same net as
+//  PB7 (I2C SDA). Driving SPI on PA5/PA6 while I2C runs on PB6/PB7 would put
+//  two peripherals on one net. See UM1956 Table 8 (SB16/SB18).
+//
+//      SCK  = PB3  (Arduino D13)   <- also the on-board user LED LD3, see below
+//      MISO = PB4  (Arduino D12)   <- connect to sensor SDO
+//      MOSI = PB5  (Arduino D11)   <- connect to sensor SDI
+//  These three are the STM32duino defaults (core falls back to Arduino pin
+//  numbers 13/12/11), so plain SPI.begin() selects them.
+#define BARO_SPI_CS_PIN         PA11    // Arduino D10, core default PIN_SPI_SS
+
+//  MS5611 accepts SPI mode 0 and mode 3, up to 20 MHz (datasheet p.5/p.6).
+//  The library hardcodes mode 0; 1 MHz is its default and is plenty here.
+#define BARO_SPI_CLOCK_HZ       1000000UL
+
+// ---------------------------------------------------------------------------
+//  Error indicator LEDs - one per sensor
+// ---------------------------------------------------------------------------
+//  IMPORTANT: the NUCLEO-F042K6 has exactly ONE user-controllable LED, and it
+//  is unusable for this project:
+//
+//    LD1 (COM, tricolor) - driven by the ST-LINK MCU, not by the target
+//    LD2 (PWR, red)      - hardwired to the power rail, not by the target
+//    LD3 (user, green)   - on PB3 via SB15 + R23, and PB3 is our SPI1 SCK
+//
+//  So both indicators are external LEDs (LED + ~510R to GND) on free GPIOs.
+//  PB0/PB1 are plain GPIO on this board and collide with nothing we use.
+#define LED_BARO_I2C_PIN        PB0     // Arduino D3 - error LED for the I2C sensor
+#define LED_BARO_SPI_PIN        PB1     // Arduino D6 - error LED for the SPI sensor
+
+//  How long an LED stays lit after an error is detected.
+#define LED_ERROR_HOLD_MS       1000UL
+
+// ---------------------------------------------------------------------------
+//  UART
+// ---------------------------------------------------------------------------
+//  `Serial` on this variant is USART2 (SERIAL_UART_INSTANCE 2) on PA2/PA15,
+//  which is wired to the ST-LINK Virtual COM Port. No extra wiring needed.
+#define UART_BAUD               115200UL
+
+//  Measurement report rate: 10 Hz.
+#define REPORT_PERIOD_MS        100UL
+
+// ---------------------------------------------------------------------------
+//  Acquisition
+// ---------------------------------------------------------------------------
+//  The driver's read() busy-waits through two conversions, so a poll of both
+//  sensors is the loop period, and the report deadline can only be evaluated
+//  on that grid. Keeping the cycle short keeps the 10 Hz output jitter small:
+//
+//      OSR_STANDARD   (1024) 2.28 ms/conv -> ~5 ms/sensor  -> ~11 ms cycle
+//      OSR_ULTRA_HIGH (4096) 9.04 ms/conv -> ~19 ms/sensor -> ~37 ms cycle
+//
+//  Conversion times are the datasheet p.3 maxima. OSR 1024 already resolves
+//  well under a mbar, so it is the better trade here; raise it if resolution
+//  matters more than tight report timing.
+#define BARO_OVERSAMPLING       OSR_STANDARD
+
+//  Retry interval for a sensor that failed to initialise.
+#define BARO_INIT_RETRY_MS      2000UL
+
+//  Consecutive failed polls after which the channel is torn down and taken
+//  back through the full reset + PROM/CRC handshake. Covers a sensor that was
+//  unplugged, browned out, or otherwise lost its calibration constants.
+#define BARO_REINIT_AFTER_ERRORS 10
+
+// ---------------------------------------------------------------------------
+//  Validation limits
+// ---------------------------------------------------------------------------
+//  MS5611-01BA03 operating ranges (datasheet p.2/p.4).
+#define BARO_PRESSURE_MIN_MBAR  10.0f
+#define BARO_PRESSURE_MAX_MBAR  1200.0f
+#define BARO_TEMP_MIN_C         (-40.0f)
+#define BARO_TEMP_MAX_C         85.0f
+
+//  A healthy MS5611 dithers by well under a mbar but never repeats a 24-bit
+//  reading bit-for-bit many times running. Identical pressure AND temperature
+//  this many polls in a row means the sensor stopped converting while still
+//  answering on the bus.
+#define BARO_STALE_LIMIT        32
--- a/include/ms5611_crc.h
+++ b/include/ms5611_crc.h
@@ -0,0 +1,11 @@
+#pragma once
+
+#include <stdint.h>
+
+//  MS5611 factory PROM CRC-4 (datasheet p.13, algorithm specified in AN520).
+//  `prom` holds the 8 PROM words as read from the part; word 7 carries the
+//  stored CRC in its low nibble. The array is restored before returning.
+//
+//  Deliberately free of Arduino headers so it can be built and tested on the
+//  host against the AN520 reference vector.
+bool ms5611PromCrcOk(uint16_t prom[8]);
--- a/lib/README
+++ b/lib/README
@@ -0,0 +1,46 @@
+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link into executable file.
+
+The source code of each library should be placed in an own separate directory
+("lib/your_library_name/[here are source files]").
+
+For example, see a structure of the following two libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional, custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |
+|  |- README --> THIS FILE
+|
+|- platformio.ini
+|--src
+   |- main.c
+
+and a contents of `src/main.c`:
+```
+#include <Foo.h>
+#include <Bar.h>
+
+int main (void)
+{
+  ...
+}
+
+```
+
+PlatformIO Library Dependency Finder will find automatically dependent
+libraries scanning project source files.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html
--- a/platformio.ini
+++ b/platformio.ini
@@ -0,0 +1,33 @@
+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[common]
+framework = arduino
+lib_deps =
+    SPI
+    I2C
+    https://github.com/RobTillaart/MS5611#0.5.2
+    https://github.com/RobTillaart/MS5611_SPI#0.4.3
+
+
+[env:nucleo_f042k6]
+extends = common
+platform = ststm32
+board = nucleo_f042k6
+monitor_speed = 115200
+; -flto and -fsingle-precision-constant are load-bearing, not tuning: the
+; STM32F042K6 has only 32 KB of flash and the build overflows without them.
+; Both MS5611 drivers write their compensation maths with unsuffixed double
+; literals, which would otherwise link ~5 KB of double soft-float helpers.
+build_flags =
+    -Wall
+    -Wextra
+    -flto
+    -fsingle-precision-constant
--- a/src/baro_channel.cpp
+++ b/src/baro_channel.cpp
@@ -0,0 +1,151 @@
+#include "baro_channel.h"
+
+const char *baroStatusName(BaroStatus status)
+{
+  switch (status)
+  {
+    case BaroStatus::NotInitialised: return "INIT";
+    case BaroStatus::Ok:             return "OK";
+    case BaroStatus::InitFailed:     return "ERR_INIT";
+    case BaroStatus::PromCrcError:   return "ERR_CRC";
+    case BaroStatus::ReadError:      return "ERR_READ";
+    case BaroStatus::OutOfRange:     return "ERR_RANGE";
+    case BaroStatus::Stale:          return "ERR_STALE";
+  }
+  return "ERR_UNKNOWN";
+}
+
+
+void BaroChannel::begin(uint32_t now)
+{
+  pinMode(_ledPin, OUTPUT);
+  digitalWrite(_ledPin, LOW);
+  _ledOn = false;
+
+  tryInit(now);
+}
+
+
+bool BaroChannel::tryInit(uint32_t now)
+{
+  _lastInitAttempt = now;
+  _initialised     = false;
+  _repeatCount     = 0;
+
+  if (!driverBegin())
+  {
+    fail(BaroStatus::InitFailed, now);
+    return false;
+  }
+
+  uint16_t prom[8] = { 0 };
+  driverReadProm(prom);
+
+  //  An absent part reads back as all-zero (I2C NACK) or all-ones (floating
+  //  MISO). Neither driver rejects 0xFFFF, and an all-zero PROM would satisfy
+  //  the CRC, so the calibration words are screened before the checksum runs.
+  for (uint8_t i = 1; i <= 6; i++)
+  {
+    if (prom[i] == 0x0000 || prom[i] == 0xFFFF)
+    {
+      fail(BaroStatus::PromCrcError, now);
+      return false;
+    }
+  }
+
+  if (!ms5611PromCrcOk(prom))
+  {
+    fail(BaroStatus::PromCrcError, now);
+    return false;
+  }
+
+  _initialised = true;
+  _errorStreak = 0;
+  return true;
+}
+
+
+void BaroChannel::poll(uint32_t now)
+{
+  if (!_initialised)
+  {
+    if ((uint32_t)(now - _lastInitAttempt) < BARO_INIT_RETRY_MS) return;
+    if (!tryInit(now)) return;
+  }
+
+  if (!driverRead())
+  {
+    fail(BaroStatus::ReadError, now);
+    return;
+  }
+
+  const float pressure    = driverPressure();
+  const float temperature = driverTemperature();
+
+  //  Catches the failure the drivers miss: an ADC read taken before the
+  //  conversion finished returns 0 (datasheet p.11), which the I2C driver
+  //  still reports as MS5611_READ_OK but which lands far outside this envelope.
+  if (!isfinite(pressure) || !isfinite(temperature) ||
+      pressure < BARO_PRESSURE_MIN_MBAR || pressure > BARO_PRESSURE_MAX_MBAR ||
+      temperature < BARO_TEMP_MIN_C || temperature > BARO_TEMP_MAX_C)
+  {
+    _pressure    = pressure;
+    _temperature = temperature;
+    fail(BaroStatus::OutOfRange, now);
+    return;
+  }
+
+  const bool repeated = (pressure == _pressure) && (temperature == _temperature);
+
+  _pressure    = pressure;
+  _temperature = temperature;
+
+  if (repeated)
+  {
+    if (_repeatCount < BARO_STALE_LIMIT) _repeatCount++;
+  }
+  else
+  {
+    _repeatCount = 0;
+  }
+
+  if (_repeatCount >= BARO_STALE_LIMIT)
+  {
+    fail(BaroStatus::Stale, now);
+    return;
+  }
+
+  _status      = BaroStatus::Ok;
+  _errorStreak = 0;
+}
+
+
+void BaroChannel::fail(BaroStatus status, uint32_t now)
+{
+  _status = status;
+  _errorCount++;
+
+  _ledOn    = true;
+  _ledOffAt = now + LED_ERROR_HOLD_MS;
+  digitalWrite(_ledPin, HIGH);
+
+  if (_errorStreak < BARO_REINIT_AFTER_ERRORS) _errorStreak++;
+
+  if (_errorStreak >= BARO_REINIT_AFTER_ERRORS)
+  {
+    _initialised     = false;
+    _errorStreak     = 0;
+    _lastInitAttempt = now;
+  }
+}
+
+
+void BaroChannel::updateLed(uint32_t now)
+{
+  //  Signed difference so the comparison survives the millis() rollover.
+  if (_ledOn && (int32_t)(now - _ledOffAt) >= 0)
+  {
+    _ledOn = false;
+    digitalWrite(_ledPin, LOW);
+  }
+}
--- a/src/baro_i2c.cpp
+++ b/src/baro_i2c.cpp
@@ -0,0 +1,46 @@
+#include <Wire.h>
+#include <MS5611.h>
+
+#include "baro_channel.h"
+
+namespace {
+
+class BaroI2cChannel : public BaroChannel
+{
+public:
+  BaroI2cChannel()
+    : BaroChannel("i2c", LED_BARO_I2C_PIN), _driver(BARO_I2C_ADDRESS, &Wire) {}
+
+protected:
+  bool driverBegin() override
+  {
+    if (!_busStarted)
+    {
+      Wire.begin();
+      _busStarted = true;
+    }
+
+    _driver.setOversampling(BARO_OVERSAMPLING);
+
+    return _driver.begin();
+  }
+
+  void driverReadProm(uint16_t *prom) override
+  {
+    for (uint8_t i = 0; i < 8; i++) prom[i] = _driver.getProm(i);
+  }
+
+  bool  driverRead() override        { return _driver.read() == MS5611_READ_OK; }
+  float driverPressure() override    { return _driver.getPressure(); }
+  float driverTemperature() override { return _driver.getTemperature(); }
+
+private:
+  MS5611 _driver;
+  bool   _busStarted = false;
+};
+
+BaroI2cChannel instance;
+
+}  // namespace
+
+BaroChannel &baroI2cChannel() { return instance; }
--- a/src/baro_spi.cpp
+++ b/src/baro_spi.cpp
@@ -0,0 +1,49 @@
+#include <SPI.h>
+#include <MS5611_SPI.h>
+
+#include "baro_channel.h"
+
+namespace {
+
+class BaroSpiChannel : public BaroChannel
+{
+public:
+  BaroSpiChannel()
+    : BaroChannel("spi", LED_BARO_SPI_PIN), _driver(BARO_SPI_CS_PIN, &SPI) {}
+
+protected:
+  bool driverBegin() override
+  {
+    if (!_busStarted)
+    {
+      SPI.begin();
+      _busStarted = true;
+    }
+
+    //  Both must precede begin(), which re-applies the stored SPI speed and
+    //  then talks to the part.
+    _driver.setSPIspeed(BARO_SPI_CLOCK_HZ);
+    _driver.setOversampling(BARO_OVERSAMPLING);
+
+    return _driver.begin();
+  }
+
+  void driverReadProm(uint16_t *prom) override
+  {
+    for (uint8_t i = 0; i < 8; i++) prom[i] = _driver.getProm(i);
+  }
+
+  bool  driverRead() override        { return _driver.read() == MS5611_READ_OK; }
+  float driverPressure() override    { return _driver.getPressure(); }
+  float driverTemperature() override { return _driver.getTemperature(); }
+
+private:
+  MS5611_SPI _driver;
+  bool       _busStarted = false;
+};
+
+BaroSpiChannel instance;
+
+}  // namespace
+
+BaroChannel &baroSpiChannel() { return instance; }
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -0,0 +1,93 @@
+#include <Arduino.h>
+
+#include "baro_channel.h"
+#include "config.h"
+
+namespace {
+
+uint32_t nextReportAt = 0;
+
+//  Serial.print(float, digits) resolves to Print::print(double, int), which
+//  would pull the double-precision soft-float routines into a 32 KB image.
+//  Formatting from a scaled integer keeps the whole build single-precision.
+void printFixed2(float value)
+{
+  if (!isfinite(value))
+  {
+    Serial.print(F("nan"));
+    return;
+  }
+
+  int32_t hundredths = (int32_t)(value * 100.0f + (value < 0.0f ? -0.5f : 0.5f));
+
+  if (hundredths < 0)
+  {
+    Serial.print('-');
+    hundredths = -hundredths;
+  }
+
+  const int32_t fraction = hundredths % 100;
+
+  Serial.print(hundredths / 100);
+  Serial.print('.');
+  if (fraction < 10) Serial.print('0');
+  Serial.print(fraction);
+}
+
+void printChannel(BaroChannel &channel)
+{
+  Serial.print(',');
+  Serial.print(baroStatusName(channel.status()));
+  Serial.print(',');
+  printFixed2(channel.pressure());
+  Serial.print(',');
+  printFixed2(channel.temperature());
+}
+
+}  // namespace
+
+
+void setup()
+{
+  Serial.begin(UART_BAUD);
+
+  const uint32_t now = millis();
+  baroI2cChannel().begin(now);
+  baroSpiChannel().begin(now);
+
+  Serial.println();
+  Serial.println(F("ms,i2c_status,i2c_p_mbar,i2c_t_c,spi_status,spi_p_mbar,spi_t_c"));
+
+  nextReportAt = millis() + REPORT_PERIOD_MS;
+}
+
+
+void loop()
+{
+  uint32_t now = millis();
+
+  baroI2cChannel().poll(now);
+  baroSpiChannel().poll(now);
+
+  //  poll() blocks for the ADC conversions, so re-read the clock before the
+  //  LED and report deadlines are evaluated.
+  now = millis();
+
+  baroI2cChannel().updateLed(now);
+  baroSpiChannel().updateLed(now);
+
+  if ((int32_t)(now - nextReportAt) < 0) return;
+
+  //  Advancing by a fixed period keeps the long-run average at exactly 10 Hz
+  //  instead of drifting by one loop iteration per report.
+  nextReportAt += REPORT_PERIOD_MS;
+
+  //  Unless a stall put us more than a full period behind, in which case the
+  //  grid is re-based rather than emitting a burst of catch-up lines.
+  if ((int32_t)(now - nextReportAt) >= 0) nextReportAt = now + REPORT_PERIOD_MS;
+
+  Serial.print(now);
+  printChannel(baroI2cChannel());
+  printChannel(baroSpiChannel());
+  Serial.println();
+}
--- a/src/ms5611_crc.cpp
+++ b/src/ms5611_crc.cpp
@@ -0,0 +1,27 @@
+#include "ms5611_crc.h"
+
+bool ms5611PromCrcOk(uint16_t prom[8])
+{
+  const uint16_t crcRead = prom[7];
+  uint16_t remainder = 0;
+
+  //  The 4 CRC bits live in the low nibble of word 7 and must read as zero
+  //  while the remainder is computed, so the whole low byte is masked off.
+  prom[7] &= 0xFF00;
+
+  for (uint8_t i = 0; i < 16; i++)
+  {
+    if (i & 1) remainder ^= (uint16_t)(prom[i >> 1] & 0x00FF);
+    else       remainder ^= (uint16_t)(prom[i >> 1] >> 8);
+
+    for (uint8_t bit = 8; bit > 0; bit--)
+    {
+      remainder = (remainder & 0x8000) ? (uint16_t)((remainder << 1) ^ 0x3000)
+                                       : (uint16_t)(remainder << 1);
+    }
+  }
+
+  prom[7] = crcRead;
+
+  return (uint16_t)((remainder >> 12) & 0x000F) == (uint16_t)(crcRead & 0x000F);
+}
--- a/test/README
+++ b/test/README
@@ -0,0 +1,11 @@
+
+This directory is intended for PlatformIO Test Runner and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PlatformIO Unit Testing:
+- https://docs.platformio.org/en/latest/advanced/unit-testing/index.html