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main.c

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#include <hardware/structs/io_bank0.h>
#include <pico/time.h>
#include <stdint.h>
#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/gpio.h"
#include "hardware/spi.h"

#define SPI_PORT spi1

#define MOSI 11
#define MISO 8
#define CS 9
#define CLK 10

#define READ_BIT 0x80
#define WRITE_BIT 0x00
#define MB_BIT 0x40

#define DATAX0 0x32

#define POWERCTL 0x2D
#define MEASURE_BIT 0x08

#define DATA_FORMAT 0x30
#define ADXL345_RANGE_PM_2G 0x00
#define ADXL345_RANGE_PM_4G 0x01
#define ADXL345_RANGE_PM_8G 0x02
#define ADXL345_RANGE_PM_16G 0x03
#define ADXL345_LEFT_JUSTIFY 0x04
#define ADXL345_SCALE_FACTOR 0.0039

struct Config {
	uint scale;
	bool full_resolution;
};

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

void read_registers(uint8_t reg_address, uint8_t *buf, uint16_t len) {
	reg_address = READ_BIT | reg_address;

	cs_select();
	spi_write_blocking(SPI_PORT, &reg_address, 1);
	spi_read_blocking(SPI_PORT, 0, buf, len);
	cs_deselect();
}

void write_register(uint8_t reg_address, uint8_t byte) {
	uint8_t buf[2] = {WRITE_BIT | reg_address,byte};

	cs_select();
	spi_write_blocking(SPI_PORT, buf, 2);
	cs_deselect();
}

void read_acceleration(uint16_t accel[3]) {
	uint8_t reg_address = MB_BIT | DATAX0;
	uint8_t buffer[6];

	read_registers(reg_address, buffer, 6);

  for (int i = 0; i < 3; i++) {
      accel[i] = (uint16_t)(buffer[(i * 2) + 1] << 8) + buffer[i * 2];
  }
}

void acceleration_in_g(uint16_t accel[3], float accel_unit_g[3], struct Config conf) {
	uint bit_mask = 0x3FF;
	uint msb = 0x200;

	if (conf.full_resolution == true) {
		switch (conf.scale) {
			case 16: 
				bit_mask += 0x1C00;
				msb <<= 3;
			case 8: 
				bit_mask += 0xC00;
				msb <<= 2;
			case 4:
				bit_mask += 0x400;
				msb <<= 1;
		}
	}

	for (int i = 0; i < 3; i++) {
		if ( (accel[i] & msb) != 0 ) {
			accel_unit_g[i] = (float)( (accel[i] ^ 0xffff) + 1 ) * ADXL345_SCALE_FACTOR;
			accel_unit_g[i] *= -1;
		} else {
			accel_unit_g[i] = (float)( accel[i] ) * ADXL345_SCALE_FACTOR;
		}
	}
}

int main() {
    stdio_init_all();

    spi_init(SPI_PORT, 2000*1000);
		spi_set_format(SPI_PORT, 8, SPI_CPOL_1, SPI_CPHA_1, SPI_MSB_FIRST);

		gpio_set_function(MOSI, GPIO_FUNC_SPI);
		gpio_set_function(MISO, GPIO_FUNC_SPI);
		gpio_set_function(CLK, GPIO_FUNC_SPI);

    gpio_init(CS);
    gpio_set_dir(CS, GPIO_OUT);
    gpio_put(CS, 1);

		uint16_t accel[3] = {0x67, 0x67, 0x67};
		float float_accel[3];

		uint8_t id;
    read_registers(0x00, &id, 1);
		write_register(POWERCTL, MEASURE_BIT);

		struct Config config = {.scale = 16, .full_resolution = false};
		write_register(DATA_FORMAT, ADXL345_RANGE_PM_16G);

    while (true) {
				read_acceleration(accel);
				acceleration_in_g(accel, float_accel, config);
				printf("%f %f %f\n", float_accel[0],float_accel[1],float_accel[2]);
				//printf("%x %x %x\n\n", accel[0],accel[1],accel[2]);
        sleep_ms(100);
    }
}