mpu9250_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 MPU9250 MEMS accelerometer and gyroscope.
   Ignores the magnetometer, that is left as a exercise for the reader.

   This is taking to simple approach of simply reading registers. It's perfectly
   possible to link up an interrupt line and set things up to read from the
   inbuilt FIFO to make it more useful.

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

   You will need to use a level shifter on the I2C lines if you want to run the
   board at 5v.

   Connections on Raspberry Pi Pico board and a generic MPU9250 board, other
   boards may vary.

   GPIO 4 (pin 6) MISO/spi0_rx-> ADO on MPU9250 board
   GPIO 5 (pin 7) Chip select -> NCS on MPU9250 board
   GPIO 6 (pin 9) SCK/spi0_sclk -> SCL on MPU9250 board
   GPIO 7 (pin 10) MOSI/spi0_tx -> SDA on MPU9250 board
   3.3v (pin 36) -> VCC on MPU9250 board
   GND (pin 38)  -> GND on MPU9250 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.
   The particular device used here uses the same pins for I2C and SPI, hence the
   using of I2C names
*/

#define PIN_MISO 4
#define PIN_CS   5
#define PIN_SCK  6
#define PIN_MOSI 7

#define SPI_PORT spi0
#define READ_BIT 0x80

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

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

static void mpu9250_reset() {
    // Two byte reset. First byte register, second byte data
    // There are a load more options to set up the device in different ways that could be added here
    uint8_t buf[] = {0x6B, 0x00};
    cs_select();
    spi_write_blocking(SPI_PORT, buf, 2);
    cs_deselect();
}


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_PORT, &reg, 1);
    sleep_ms(10);
    spi_read_blocking(SPI_PORT, 0, buf, len);
    cs_deselect();
    sleep_ms(10);
}


static void mpu9250_read_raw(int16_t accel[3], int16_t gyro[3], int16_t *temp) {
    uint8_t buffer[6];

    // Start reading acceleration registers from register 0x3B for 6 bytes
    read_registers(0x3B, buffer, 6);

    for (int i = 0; i < 3; i++) {
        accel[i] = (buffer[i * 2] << 8 | buffer[(i * 2) + 1]);
    }

    // Now gyro data from reg 0x43 for 6 bytes
    read_registers(0x43, buffer, 6);

    for (int i = 0; i < 3; i++) {
        gyro[i] = (buffer[i * 2] << 8 | buffer[(i * 2) + 1]);;
    }

    // Now temperature from reg 0x41 for 2 bytes
    read_registers(0x41, buffer, 2);

    *temp = buffer[0] << 8 | buffer[1];
}

int main() {
    stdio_init_all();

    printf("Hello, MPU9250! Reading raw data from registers via SPI...\n");

    // This example will use SPI0 at 0.5MHz.
    spi_init(SPI_PORT, 500 * 1000);
    gpio_set_function(PIN_MISO, GPIO_FUNC_SPI);
    gpio_set_function(PIN_SCK, GPIO_FUNC_SPI);
    gpio_set_function(PIN_MOSI, GPIO_FUNC_SPI);
    // Make the SPI pins available to picotool
    bi_decl(bi_3pins_with_func(PIN_MISO, PIN_MOSI, PIN_SCK, GPIO_FUNC_SPI));

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

    mpu9250_reset();

    // See if SPI is working - interrograte the device for its I2C ID number, should be 0x71
    uint8_t id;
    read_registers(0x75, &id, 1);
    printf("I2C address is 0x%x\n", id);

    int16_t acceleration[3], gyro[3], temp;

    while (1) {
        mpu9250_read_raw(acceleration, gyro, &temp);

        // These are the raw numbers from the chip, so will need tweaking to be really useful.
        // See the datasheet for more information
        printf("Acc. X = %d, Y = %d, Z = %d\n", acceleration[0], acceleration[1], acceleration[2]);
        printf("Gyro. X = %d, Y = %d, Z = %d\n", gyro[0], gyro[1], gyro[2]);
        // Temperature is simple so use the datasheet calculation to get deg C.
        // Note this is chip temperature.
        printf("Temp. = %f\n", (temp / 340.0) + 36.53);

        sleep_ms(100);
    }

    return 0;
}