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README.md
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# Barometric pressure test
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## Goals
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1. Read barometric pressure from two sensors. One connected to the MCU with SPI and the other connected with I2C bus.
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2. The reading must be done in real time and data should be checked for validity.
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3. The output should be sent to a serial port.
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4. In case of reading error the LED should be turned on for 1 second. Each sensor has its own LED.
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## Hardware
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- MCU - STM32F042K6T6 (Nucleo-32, board MB1180) - 32 KB flash, 6 KB RAM
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- Barometric pressure sensor - 2x GY-63 (MS5611)
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## Wiring
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Pin assignments follow ST UM1956 Table 10 and the MB1180 C.2 schematic, and match
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the STM32duino defaults for this variant, so `Wire.begin()` and `SPI.begin()` need
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no explicit pin overrides.
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Vector source: [`docs/wiring.svg`](docs/wiring.svg). Pins there are grouped by
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function, not by physical header order.
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The same wiring drawn physically, with the real header order from UM1956
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Table 10, so it is clear which hole each jumper goes into:
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Vector source: [`docs/wiring-physical.svg`](docs/wiring-physical.svg). Note that
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SPI is split across both connectors: MOSI, MISO and CS are on CN3, while SCK is
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CN4 pin 15.
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### Baro0 - I2C (GY-63 <=> STM32F042)
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| GY-63 | Nucleo | STM32 | Note |
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|-------|--------|-------|------|
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| VCC | 3V3 | - | |
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| GND | GND | - | |
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| PS | 3.3V | - | high selects I2C |
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| SDA | A4 | PB7 | `PIN_WIRE_SDA`, reaches the A4 pad through SB18 |
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| SCL | A5 | PB6 | `PIN_WIRE_SCL`, reaches the A5 pad through SB16 |
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| CSB | GND | - | sets address `0x77`; tie to VCC for `0x76` |
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The MS5611 address is `1110 11Cx`, where C is the **complement** of CSB
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(datasheet p.12). `CSB` must not be left floating. If you strap it high, change
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`BARO_I2C_ADDRESS` in [`include/config.h`](include/config.h).
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### Baro1 - SPI (GY-63 <=> STM32F042)
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| GY-63 | Nucleo | STM32 | Note |
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|-------|--------|-------|------|
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| VCC | 3V3 | - | |
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| GND | GND | - | |
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| PS | GND | - | low selects SPI |
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| SCLK | D13 | PB3 | SPI1_SCK |
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| SDI | D11 | PB5 | SPI1_MOSI |
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| SDO | D12 | PB4 | SPI1_MISO |
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| CSB | D10 | PA11 | chip select, `BARO_SPI_CS_PIN` |
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SPI1 has to stay on this pin group. The alternative group (PA5/PA6/PA7) is
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unusable while I2C is active: with the factory-default solder bridges SB16 and
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SB18 closed, PA6 shares a net with PB6 (SCL) and PA5 shares a net with PB7 (SDA).
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### Error LEDs
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Two external LEDs, one per sensor, each in series with roughly 510 R to GND.
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| Signal | Nucleo | STM32 |
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|--------|--------|-------|
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| I2C sensor error | D3 | PB0 |
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| SPI sensor error | D6 | PB1 |
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Both pins are set in [`include/config.h`](include/config.h).
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## Build and run
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```sh
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pio run # build
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pio run -t upload # flash over ST-LINK
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pio device monitor # 115200 baud, ST-LINK Virtual COM Port
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```
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`Serial` is USART2 on PA2/PA15, wired to the ST-LINK VCP, so no USB-serial
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adapter is needed.
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The image is close to the flash ceiling (about 89% of 32 KB). `-flto` and
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`-fsingle-precision-constant` in [`platformio.ini`](platformio.ini) are required,
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not cosmetic: both MS5611 drivers write their compensation maths with unsuffixed
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double literals, which otherwise links roughly 5 KB of double soft-float helpers
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and overflows flash.
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## Output
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CSV at 10 Hz on the serial port, with a header line printed once at startup:
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```
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ms,i2c_status,i2c_p_mbar,i2c_t_c,spi_status,spi_p_mbar,spi_t_c
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1043,OK,1013.24,24.31,OK,1013.19,24.44
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1143,OK,1013.25,24.31,ERR_READ,1013.19,24.44
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```
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Pressure is in mbar, temperature in degrees C, both to two decimals. The status
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column is authoritative: when it is not `OK`, the two values next to it are the
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last ones successfully computed, not fresh readings.
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| Status | Meaning |
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|--------|---------|
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| `INIT` | not initialised yet |
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| `OK` | reading passed every check |
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| `ERR_INIT` | reset / PROM handshake did not complete |
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| `ERR_CRC` | factory calibration failed CRC-4, or is all-zero / all-ones |
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| `ERR_READ` | driver reported a bus or ADC failure |
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| `ERR_RANGE` | value outside the MS5611 operating envelope |
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| `ERR_STALE` | sensor still answers but stopped producing new samples |
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Sensors are polled continuously, as fast as the ADC conversions allow (roughly
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90 Hz per sensor at OSR 1024); the serial report is throttled to 10 Hz.
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## Validation
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Neither driver library detects every failure on its own, so the checks live in
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[`BaroChannel`](src/baro_channel.cpp):
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- **PROM CRC-4** at init, per datasheet p.13 / AN520. Neither library verifies it.
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- **All-zero / all-ones PROM screening**, because an absent part reads back as
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one of those, `0xFFFF` is accepted by both libraries' `reset()`, and an
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all-zero PROM satisfies the CRC.
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- **Driver return code** on every read.
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- **Range check** against 10..1200 mbar and -40..+85 C. This is the one that
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catches the failure the I2C driver misses entirely: an ADC read taken before
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the conversion completes returns 0 (datasheet p.11), and `MS5611::read()`
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still reports `MS5611_READ_OK` for it.
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- **Stale detection** - bit-identical pressure *and* temperature for 32
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consecutive polls means the part stopped converting while still acknowledging.
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Any failure lights that sensor's LED for 1 second (non-blocking, extended if
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further errors follow). After 10 consecutive failures the channel is taken back
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through the full reset and PROM handshake, so a sensor that was unplugged and
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reconnected recovers on its own.
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The CRC-4 implementation was verified differentially on the host against the
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independent NuttX reference over 200000 randomised PROM images.
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## References
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- [Barometric Pressure Sensor MS5611](docs/datasheets/ENG_DS_MS5611-01BA03_B3.pdf)
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- [STM32F042K6T6 Nucleo](docs/datasheets/DS_stm32f042k6.pdf)
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- [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
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