barometer.cpp

#include "barometer.h"
#include <Arduino.h>
#include <Wire.h>
#include <Adafruit_DPS310.h>

static Adafruit_DPS310 dps;

// The part's operating range, from Adafruit_DPS310_Pressure::getSensor().
static const float DPS310_MIN_HPA = 300.0f;
static const float DPS310_MAX_HPA = 1200.0f;

// begin_I2C() leaves the DPS310 at 64 Hz / 64x oversampling and the part cannot
// actually run that. One conversion costs 20 + 16 * oversampling tenths of a
// millisecond - the datasheet formula, spelled out in Infineon's own driver as
// calcBusyTime() - so 64x is 104 ms, and continuous mode alternates pressure
// with temperature, which puts fresh pressure roughly 210 ms apart. Polled from
// a 20 ms control loop that is one poll in ten, and initSensors() averages one
// or two samples for its ground reference instead of the sixteen it asks for.
//
// 8x converts in 14.8 ms, so 32 Hz sits at about half the part's duty budget,
// and temperature only feeds the pressure compensation and barely moves, so it
// drops to 8 Hz at 1x. The cost is pressure noise, which sensors.cpp already
// low-passes; the gain is ~90 ms of lag out of a loop that differentiates
// altitude to get climb rate.
static const dps310_rate_t PRESSURE_RATE = DPS310_32HZ;
static const dps310_oversample_t PRESSURE_OVERSAMPLE = DPS310_8SAMPLES;
static const dps310_rate_t TEMPERATURE_RATE = DPS310_8HZ;
static const dps310_oversample_t TEMPERATURE_OVERSAMPLE = DPS310_1SAMPLE;

// There is deliberately no "wait for the first sample" loop in here. It looks
// like the obvious guard and it would be dead code: begin_I2C() cannot return
// true until _init()'s own wait has seen MEAS_CFG's PRS_RDY set, and none of the
// four calls after it read PRS_B2..B0, which is the only thing that clears the
// bit. A poll would exit on its first iteration on the flag left behind by the
// 64x conversion _init() already blocked for - it would report "the part
// converts" while proving nothing about the settings we just wrote.
//
// The case it looks like it catches - a chip that converts once and then quits -
// is caught a layer up instead, and caught properly: initSensors() averages
// sixteen reads for its ground reference and clears baroUp if none of them land
// (sensors.cpp), so a part that stopped gets about 320 ms to prove otherwise and
// the aircraft refuses to arm when it does not.
bool initBarometer() {
    Wire.begin();

    // Caveat on the no-hang rule: Adafruit_DPS310::_init() spins in its own
    // unbounded `while (!temperatureAvailable() || !pressureAvailable())`, so
    // begin_I2C() can still sit here forever. Bounding that would mean
    // vendoring the library. Nothing this function adds can block.
    if (!dps.begin_I2C()) {
        Serial.println("DPS310 did not answer on I2C");
        return false;
    }

    dps.setMode(DPS310_IDLE);
    dps.configurePressure(PRESSURE_RATE, PRESSURE_OVERSAMPLE);
    dps.configureTemperature(TEMPERATURE_RATE, TEMPERATURE_OVERSAMPLE);
    dps.setMode(DPS310_CONT_PRESTEMP);
    return true;
}

// Adafruit_DPS310::getEvents() ends in an unconditional `return true;` - it
// hands back whatever _read() last left in the object, so on its own it cannot
// tell a live chip from one that stopped answering. The two checks around it are
// what make a bad read detectable:
//
//   pressureAvailable() is the MEAS_CFG "new pressure data" bit. The sensor
//   converts at 32 Hz against a 50 Hz poll, so this is clear on roughly a third
//   of calls in normal flight - a false return means "nothing new this cycle",
//   not "the barometer is broken".
//
//   The range is the DPS310's own operating span, 300..1200 hPa, straight out of
//   the library's getSensor() bounds. Garbage off a dead I2C bus lands outside it.
bool readBarometer(float *pressure_hPa) {
    if (!dps.pressureAvailable()) {
        return false;
    }

    sensors_event_t temp_event, pressure_event;
    dps.getEvents(&temp_event, &pressure_event);

    float pressure = pressure_event.pressure;
    if (!(pressure >= DPS310_MIN_HPA && pressure <= DPS310_MAX_HPA)) {
        return false;   // the >= form also rejects NaN
    }

    *pressure_hPa = pressure;
    return true;
}