baro-pressure-readings/README.md

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.

Vector source: 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:

Vector source: 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.

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.

Build and run

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 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:

• 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
• STM32F042K6T6 Nucleo
• STM32 Nucleo-32 boards (MB1180) - UM1956, LED and connector tables