lps25.cpp

Go to the documentation of this file.

/**
 * @file        lps25.cpp
 * @brief       Driver for the ST LPS25HB pressure sensor
 *
 * At the moment uses bit-banged I2C driver
 *
 * @date February 6, 2020
 * @author Matthew Kennedy, (c) 2020
 */
 
#include "pch.h"
 
#include "lps25.h"
 
static constexpr uint8_t addr = 0x5C;
static constexpr uint8_t expectedWhoAmILps22 = 0xB1;
static constexpr uint8_t expectedWhoAmILps25 = 0xBD;
 
// Control register 1
#define LPS_CR1_PD (1 << 7)
#define LPS_CR1_ODR_25hz (4 << 4)
#define LPS_CR1_BDU (1 << 2)
 
// Status register flags
#define LPS_SR_P_DA (1 << 1)    // Pressure data available
 
#define REG_WhoAmI 0x0F
 
// register address different on LPS22 vs LPS25
#define REG_Cr1_Lps22 0x10
#define REG_Cr1_Lps25 0x20
#define REG_Status 0x27
#define REG_PressureOutXl 0x28
#define REG_PressureOutL 0x29
#define REG_PressureOutH 0x2A
 
bool Lps25::init(brain_pin_e scl, brain_pin_e sda) {
    if (!m_i2c.init(scl, sda)) {
        return false;
    }
 
    // Read ident register
    auto whoAmI = m_i2c.readRegister(addr, REG_WhoAmI);
 
    switch (whoAmI)
    {
    case expectedWhoAmILps22:
        m_type = Type::Lps22;
        break;
    case expectedWhoAmILps25:
        m_type = Type::Lps25;
        break;
    default:
        // chip not detected
        return false;
    }
 
    uint8_t cr1 = 
        LPS_CR1_ODR_25hz |  // 25hz update rate
        // TODO: should bdu be set?
        LPS_CR1_BDU;        // Output registers update only when read
 
    if (m_type == Type::Lps25) {
        // Set to active mode
        // this bit must be 0 on LPS22
        cr1 |= LPS_CR1_PD;
    }
 
    // Set the control registers
    m_i2c.writeRegister(addr, regCr1(), cr1);
 
    m_hasInit = true;
    return true;
}
 
expected<float> Lps25::readPressureKpa() {
    if (!m_hasInit) {
        return unexpected;
    }
 
    uint8_t buffer[4];
    // Sequential multi-byte reads need to set the high bit of the
    // register address to enable multi-byte read
    constexpr uint8_t readAddr = REG_Status | 0x80;
    m_i2c.writeRead(addr, &readAddr, 1, buffer, 4);
 
    // First check the status reg to check if there are data available
    bool hasPressure = buffer[0] & LPS_SR_P_DA;
 
    if (!hasPressure) {
        return unexpected;
    }
 
    // Glue the 3 bytes back in to a 24 bit integer
    uint32_t counts = buffer[3] << 16 | buffer[2] << 8 | buffer[1];
 
    // 4096 counts per hectopascal
    // = 40960 counts per kilopascal
    constexpr float ratio = 1 / 40960.0f;
    
    float kilopascal = counts * ratio;
 
    // Sensor limits are 26-126 kPa
    // The highest ever barometric pressure measured was only 108.3 kPa
    // The pressure at the highest altitude road (Khardung La, India/Tibet) is at 5600 meters,
    // which should have a nominal barometric pressure of around 50 kPa
    // Anything outside that range is not a place we expect your engine to run, so we assume
    // some sensing problem (sealed ECU case and high temperature?)
    if (kilopascal > 120 || kilopascal < 50) {
        return unexpected;
    }
 
    return kilopascal;
}
 
uint8_t Lps25::regCr1() const {
    switch (m_type)
    {
    case Type::Lps22:
        return REG_Cr1_Lps22;
    case Type::Lps25:
    default:
        return REG_Cr1_Lps25;
    }
}