Reading AM2320 sensor value under FreeBSD

am2320.c

#include <stdio.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>
#include <dev/iicbus/iic.h>

#define I2C_DEVICE "/dev/iic0"
#define AM2320_ADDR 0x5C

static uint16_t
_calc_crc16(const uint8_t *buf, size_t len) {
  uint16_t crc = 0xFFFF;
  while(len--) {
    crc ^= (uint16_t) *buf++;
    for (unsigned i = 0; i < 8; i++) {
      if (crc & 0x0001) {
	crc >>= 1;
	crc ^= 0xA001;
      } else {
	crc >>= 1;
      }
    }
  }

  return crc;
}

static uint16_t
_combine_bytes(uint8_t msb, uint8_t lsb)
{
  return ((uint16_t)msb << 8) | (uint16_t)lsb;
}

static int
write_bytes(int fd, int slave, int len, uint8_t *buf)
{
        struct iic_msg msg[2];
        struct iic_rdwr_data rdwr;

        msg[0].slave = slave << 1;
        msg[0].flags = IIC_M_WR;
        msg[0].len = len;
        msg[0].buf = buf;

        rdwr.msgs = msg;
        rdwr.nmsgs = 1;

        return (ioctl(fd, I2CRDWR, &rdwr));
}

static int
read_bytes(int fd, int slave, int off, int len, uint8_t *buf)
{
        struct iic_msg msg[2];
        struct iic_rdwr_data rdwr;
        uint8_t offset[2];

        offset[0] = 0x15;
        offset[1] = off;
        msg[0].slave = slave << 1;
        msg[0].flags = IIC_M_WR;
        msg[0].len = sizeof( offset );
        msg[0].buf = offset;

        msg[1].slave = slave << 1;
        msg[1].flags = IIC_M_RD;
        msg[1].len = len;
        msg[1].buf = buf;

        rdwr.msgs = msg;
        rdwr.nmsgs = 2;

        return (ioctl(fd, I2CRDWR, &rdwr));
}

int
am2320(float *out_temperature, float *out_humidity)
{
  int fd;
  uint8_t data[8] = {0};

  fd = open(I2C_DEVICE, O_RDWR);
  if (fd < 0)
    return 1;

  /* wake AM2320 up, goes to sleep to not warm up and
   * affect the humidity sensor
   */
  data[0] = 0;
  write_bytes(fd, AM2320_ADDR, 1, data);

  usleep(1000); /* at least 0.8ms, at most 3ms */

  /* write at addr 0x03, start reg = 0x00, num regs = 0x04 */
  data[0] = 0x03;
  data[1] = 0x00;
  data[2] = 0x04;

  if (write_bytes(fd, AM2320_ADDR, 3, data) < 0)
    return 3;

  /* wait for AM2320 */
  usleep(1600); /* Wait atleast 1.5ms */

  /*
   * Read out 8 bytes of data
   * Byte 0: Should be Modbus function code 0x03
   * Byte 1: Should be number of registers to read (0x04)
   * Byte 2: Humidity msb
   * Byte 3: Humidity lsb
   * Byte 4: Temperature msb
   * Byte 5: Temperature lsb
   * Byte 6: CRC lsb byte
   * Byte 7: CRC msb byte
   */
  if (read_bytes(fd, AM2320_ADDR, 0, 8, data) < 0)
    return 4;

  close(fd);

  // printf("[0x%02x 0x%02x  0x%02x 0x%02x  0x%02x 0x%02x  0x%02x 0x%02x]\n", data[0], data[1], data[2], data[3], data[4], data[5], data[6], data[7] );

  /* Check data[0] and data[1] */
  if (data[0] != 0x03 || data[1] != 0x04)
    return 9;

  /* Check CRC */
  uint16_t crcdata = _calc_crc16(data, 6);
  uint16_t crcread = _combine_bytes(data[7], data[6]);
  if (crcdata != crcread)
    return 10;

  uint16_t temp16 = _combine_bytes(data[4], data[5]);
  uint16_t humi16 = _combine_bytes(data[2], data[3]);
  //printf("temp=%u 0x%04x  hum=%u 0x%04x\n", temp16, temp16, humi16, humi16);

  /* Temperature resolution is 16Bit,
   * temperature highest bit (Bit15) is equal to 1 indicates a
   * negative temperature, the temperature highest bit (Bit15)
   * is equal to 0 indicates a positive temperature;
   * temperature in addition to the most significant bit (Bit14 ~ Bit0)
   *  indicates the temperature sensor string value.
   * Temperature sensor value is a string of 10 times the
   * actual temperature value.
   */
  if (temp16 & 0x8000)
    temp16 = -(temp16 & 0x7FFF);

  *out_temperature = (float)temp16 / 10.0;
  *out_humidity = (float)humi16 / 10.0;

  return 0;
}

int main(void) {
  float temp, humi;

  int ret = am2320(&temp, &humi);
  if (ret) {
    printf("Err=%d\n", ret);
    return ret;
  }

  printf( "Temperature %.1f [C]\n", temp);
  printf( "Humidity    %.1f [%%]\n", humi);

  return 0;
}