bme280_spi.c

/**
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <stdio.h>
#include <string.h>
#include "pico/stdlib.h"
#include "pico/binary_info.h"
#include "hardware/spi.h"

/* Example code to talk to a bme280 humidity/temperature/pressure sensor.

   NOTE: Ensure the device is capable of being driven at 3.3v NOT 5v. The Pico
   GPIO (and therefore SPI) cannot be used at 5v.

   You will need to use a level shifter on the SPI lines if you want to run the
   board at 5v.

   Connections on Raspberry Pi Pico board and a generic bme280 board, other
   boards may vary.

   GPIO 16 (pin 21) MISO/spi0_rx-> SDO/SDO on bme280 board
   GPIO 17 (pin 22) Chip select -> CSB/!CS on bme280 board
   GPIO 18 (pin 24) SCK/spi0_sclk -> SCL/SCK on bme280 board
   GPIO 19 (pin 25) MOSI/spi0_tx -> SDA/SDI on bme280 board
   3.3v (pin 36) -> VCC on bme280 board
   GND (pin 38)  -> GND on bme280 board

   Note: SPI devices can have a number of different naming schemes for pins. See
   the Wikipedia page at https://en.wikipedia.org/wiki/Serial_Peripheral_Interface
   for variations.

   This code uses a bunch of register definitions, and some compensation code derived
   from the Bosch datasheet which can be found here.
   https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bme280-ds002.pdf
*/

#define READ_BIT 0x80

int32_t t_fine;

uint16_t dig_T1;
int16_t dig_T2, dig_T3;
uint16_t dig_P1;
int16_t dig_P2, dig_P3, dig_P4, dig_P5, dig_P6, dig_P7, dig_P8, dig_P9;
uint8_t dig_H1, dig_H3;
int8_t dig_H6;
int16_t dig_H2, dig_H4, dig_H5;

/* The following compensation functions are required to convert from the raw ADC
data from the chip to something usable. Each chip has a different set of
compensation parameters stored on the chip at point of manufacture, which are
read from the chip at startup and used in these routines.
*/
int32_t compensate_temp(int32_t adc_T) {
    int32_t var1, var2, T;
    var1 = ((((adc_T >> 3) - ((int32_t) dig_T1 << 1))) * ((int32_t) dig_T2)) >> 11;
    var2 = (((((adc_T >> 4) - ((int32_t) dig_T1)) * ((adc_T >> 4) - ((int32_t) dig_T1))) >> 12) * ((int32_t) dig_T3))
            >> 14;

    t_fine = var1 + var2;
    T = (t_fine * 5 + 128) >> 8;
    return T;
}

uint32_t compensate_pressure(int32_t adc_P) {
    int32_t var1, var2;
    uint32_t p;
    var1 = (((int32_t) t_fine) >> 1) - (int32_t) 64000;
    var2 = (((var1 >> 2) * (var1 >> 2)) >> 11) * ((int32_t) dig_P6);
    var2 = var2 + ((var1 * ((int32_t) dig_P5)) << 1);
    var2 = (var2 >> 2) + (((int32_t) dig_P4) << 16);
    var1 = (((dig_P3 * (((var1 >> 2) * (var1 >> 2)) >> 13)) >> 3) + ((((int32_t) dig_P2) * var1) >> 1)) >> 18;
    var1 = ((((32768 + var1)) * ((int32_t) dig_P1)) >> 15);
    if (var1 == 0)
        return 0;

    p = (((uint32_t) (((int32_t) 1048576) - adc_P) - (var2 >> 12))) * 3125;
    if (p < 0x80000000)
        p = (p << 1) / ((uint32_t) var1);
    else
        p = (p / (uint32_t) var1) * 2;

    var1 = (((int32_t) dig_P9) * ((int32_t) (((p >> 3) * (p >> 3)) >> 13))) >> 12;
    var2 = (((int32_t) (p >> 2)) * ((int32_t) dig_P8)) >> 13;
    p = (uint32_t) ((int32_t) p + ((var1 + var2 + dig_P7) >> 4));

    return p;
}

uint32_t compensate_humidity(int32_t adc_H) {
    int32_t v_x1_u32r;
    v_x1_u32r = (t_fine - ((int32_t) 76800));
    v_x1_u32r = (((((adc_H << 14) - (((int32_t) dig_H4) << 20) - (((int32_t) dig_H5) * v_x1_u32r)) +
                   ((int32_t) 16384)) >> 15) * (((((((v_x1_u32r * ((int32_t) dig_H6)) >> 10) * (((v_x1_u32r *
                                                                                                  ((int32_t) dig_H3))
            >> 11) + ((int32_t) 32768))) >> 10) + ((int32_t) 2097152)) *
                                                 ((int32_t) dig_H2) + 8192) >> 14));
    v_x1_u32r = (v_x1_u32r - (((((v_x1_u32r >> 15) * (v_x1_u32r >> 15)) >> 7) * ((int32_t) dig_H1)) >> 4));
    v_x1_u32r = (v_x1_u32r < 0 ? 0 : v_x1_u32r);
    v_x1_u32r = (v_x1_u32r > 419430400 ? 419430400 : v_x1_u32r);

    return (uint32_t) (v_x1_u32r >> 12);
}

#ifdef PICO_DEFAULT_SPI_CSN_PIN
static inline void cs_select() {
    asm volatile("nop \n nop \n nop");
    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, 0);  // Active low
    asm volatile("nop \n nop \n nop");
}

static inline void cs_deselect() {
    asm volatile("nop \n nop \n nop");
    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, 1);
    asm volatile("nop \n nop \n nop");
}
#endif

#if defined(spi_default) && defined(PICO_DEFAULT_SPI_CSN_PIN)
static void write_register(uint8_t reg, uint8_t data) {
    uint8_t buf[2];
    buf[0] = reg & 0x7f;  // remove read bit as this is a write
    buf[1] = data;
    cs_select();
    spi_write_blocking(spi_default, buf, 2);
    cs_deselect();
    sleep_ms(10);
}

static void read_registers(uint8_t reg, uint8_t *buf, uint16_t len) {
    // For this particular device, we send the device the register we want to read
    // first, then subsequently read from the device. The register is auto incrementing
    // so we don't need to keep sending the register we want, just the first.
    reg |= READ_BIT;
    cs_select();
    spi_write_blocking(spi_default, &reg, 1);
    sleep_ms(10);
    spi_read_blocking(spi_default, 0, buf, len);
    cs_deselect();
    sleep_ms(10);
}

/* This function reads the manufacturing assigned compensation parameters from the device */
void read_compensation_parameters() {
    uint8_t buffer[26];

    read_registers(0x88, buffer, 26);

    dig_T1 = buffer[0] | (buffer[1] << 8);
    dig_T2 = buffer[2] | (buffer[3] << 8);
    dig_T3 = buffer[4] | (buffer[5] << 8);

    dig_P1 = buffer[6] | (buffer[7] << 8);
    dig_P2 = buffer[8] | (buffer[9] << 8);
    dig_P3 = buffer[10] | (buffer[11] << 8);
    dig_P4 = buffer[12] | (buffer[13] << 8);
    dig_P5 = buffer[14] | (buffer[15] << 8);
    dig_P6 = buffer[16] | (buffer[17] << 8);
    dig_P7 = buffer[18] | (buffer[19] << 8);
    dig_P8 = buffer[20] | (buffer[21] << 8);
    dig_P9 = buffer[22] | (buffer[23] << 8);

    dig_H1 = buffer[25]; // 0xA1

    read_registers(0xE1, buffer, 8);

    dig_H2 = buffer[0] | (buffer[1] << 8); // 0xE1 | 0xE2
    dig_H3 = (int8_t) buffer[2]; // 0xE3
    dig_H4 = buffer[3] << 4 | (buffer[4] & 0xf); // 0xE4 | 0xE5[3:0]
    dig_H5 = (buffer[4] >> 4) | (buffer[5] << 4); // 0xE5[7:4] | 0xE6
    dig_H6 = (int8_t) buffer[6]; // 0xE7
}

static void bme280_read_raw(int32_t *humidity, int32_t *pressure, int32_t *temperature) {
    uint8_t buffer[8];

    read_registers(0xF7, buffer, 8);
    *pressure = ((uint32_t) buffer[0] << 12) | ((uint32_t) buffer[1] << 4) | (buffer[2] >> 4);
    *temperature = ((uint32_t) buffer[3] << 12) | ((uint32_t) buffer[4] << 4) | (buffer[5] >> 4);
    *humidity = (uint32_t) buffer[6] << 8 | buffer[7];
}
#endif

int main() {
    stdio_init_all();
#if !defined(spi_default) || !defined(PICO_DEFAULT_SPI_SCK_PIN) || !defined(PICO_DEFAULT_SPI_TX_PIN) || !defined(PICO_DEFAULT_SPI_RX_PIN) || !defined(PICO_DEFAULT_SPI_CSN_PIN)
#warning spi/bme280_spi example requires a board with SPI pins
    puts("Default SPI pins were not defined");
#else

    printf("Hello, bme280! Reading raw data from registers via SPI...\n");

    // This example will use SPI0 at 0.5MHz.
    spi_init(spi_default, 500 * 1000);
    gpio_set_function(PICO_DEFAULT_SPI_RX_PIN, GPIO_FUNC_SPI);
    gpio_set_function(PICO_DEFAULT_SPI_SCK_PIN, GPIO_FUNC_SPI);
    gpio_set_function(PICO_DEFAULT_SPI_TX_PIN, GPIO_FUNC_SPI);
    // Make the SPI pins available to picotool
    bi_decl(bi_3pins_with_func(PICO_DEFAULT_SPI_RX_PIN, PICO_DEFAULT_SPI_TX_PIN, PICO_DEFAULT_SPI_SCK_PIN, GPIO_FUNC_SPI));

    // Chip select is active-low, so we'll initialise it to a driven-high state
    gpio_init(PICO_DEFAULT_SPI_CSN_PIN);
    gpio_set_dir(PICO_DEFAULT_SPI_CSN_PIN, GPIO_OUT);
    gpio_put(PICO_DEFAULT_SPI_CSN_PIN, 1);
    // Make the CS pin available to picotool
    bi_decl(bi_1pin_with_name(PICO_DEFAULT_SPI_CSN_PIN, "SPI CS"));

    // See if SPI is working - interrograte the device for its I2C ID number, should be 0x60
    uint8_t id;
    read_registers(0xD0, &id, 1);
    printf("Chip ID is 0x%x\n", id);

    read_compensation_parameters();

    write_register(0xF2, 0x1); // Humidity oversampling register - going for x1
    write_register(0xF4, 0x27);// Set rest of oversampling modes and run mode to normal

    int32_t humidity, pressure, temperature;

    while (1) {
        bme280_read_raw(&humidity, &pressure, &temperature);

        // These are the raw numbers from the chip, so we need to run through the
        // compensations to get human understandable numbers
        temperature = compensate_temp(temperature);
        pressure = compensate_pressure(pressure);
        humidity = compensate_humidity(humidity);

        printf("Humidity = %.2f%%\n", humidity / 1024.0);
        printf("Pressure = %dPa\n", pressure);
        printf("Temp. = %.2fC\n", temperature / 100.0);

        sleep_ms(1000);
    }
#endif
}