ds18b20.c

/*

   Contiki library for DS18B20 temperature sensor -
   For more details see http://xxx

   Author -
   Author -

   License - GPLv3

 */

#include "ds18b20.h"

/* probe_for_ds18b20 probes for the sensor. Returns 0 on failure, 1 on success */
/* Assumptions: only one sensor on the "1-wire bus", on port WSN_DS18B20_PORT */
/* BUG: THIS CODE DOES NOT WORK AS INTENDED! IT RETURNS "1" EVEN WHEN THERE IS NO */
/*      SENSOR CONNECTED. */

uint8_t
ds18b20_probe(void)
{
  uint8_t result = 0;

  /* Reset 1W-bus */

  /* Pull PIN low for 480 microseconds (us) */
  /* Start with setting bit DS18B20_1_PIN to 0 */
  OW_SET_PIN_LOW();
  /* then set direction to OUT by setting DS18B20_1_DDR bit to 1 */
  OW_SET_OUTPUT();
  /* Delay 480 us */
  clock_delay_usec(480);
  /* See if sensor responds. First release the bus and switch to INput mode */
  /* by setting DS18B20_1_DDR bit to 0 */
  OW_SET_INPUT();
  /* Activate internal pull-up by  setting pin to HIGH (when in INput mode) */
  /*  OW_SET_PIN_HIGH(); */
  /* Wait for the pin to go HIGH for 64 us */
  clock_delay_usec(64);
  /* Now the sensor, if present, pulls the pin LOW for 60-240 us */
  /* Detect 0 on PIND bit DS18B20_1_PIN. Invert the result so a presence (aka a 0) */
  /* sets "result" to 1 (for success) */
  result = !OW_GET_PIN_STATE();

  /* The sensor releases the pin so it goes HIGH after 240 us, add some */
  /* for the signal to stabilize, say 300 usecs to be on the safe side? */
  if(result) {
    clock_delay_usec(300);
    /* Now the bus should be HIGH again */
    result = OW_GET_PIN_STATE();
  }

  return result;
}
/* Write 1 or 0 on the bus */

void
write_bit(uint8_t bit)
{
  /* Set pin to 0 */
  OW_SET_OUTPUT();
  OW_SET_PIN_LOW();

  /* Pin should be 0 for at least 1 us */
  clock_delay_usec(2);

  /* If we're writing a 1, let interna pull-up pull the bus high */
  /* within 15 us of setting the bus to low */
  if(bit) {
    /* Internal pull-up is activated by setting direction to IN and the */
    /* setting the pin to HIGH */
    OW_SET_INPUT();
    OW_SET_PIN_HIGH();
  }
  /* OK, now the bus is either LOW, or pulled HIGH by the internal pull-up */
  /* Let this state remain for 60 us, then release the bus */
  clock_delay_usec(60);

  /* Release the bus */
  OW_SET_PIN_HIGH();
  OW_SET_INPUT();

  /* Allow > 1 us between read/write operations */
  clock_delay_usec(2);
}
/*  */
/* Read one bit of information from the bus, and return it as 1 or 0 */
/*  */

uint8_t
read_bit(void)
{
  uint8_t bit = 0;

  /* Set pin to 0 */
  OW_SET_OUTPUT();
  OW_SET_PIN_LOW();

  /* Pin should be 0 for at least 1 us */
  clock_delay_usec(2);

  /* Now read the bus, start by setting in/out direction and activating internal */
  /* pull-up resistor */
  OW_SET_INPUT();
  OW_SET_PIN_HIGH();

  /* ds18b20 either keeps the pin down or releases the bus and the */
  /* bus then goes high because of the interna pull-up resistor */
  /* Check whichever happens before 15 us has passed */
  clock_delay_usec(15 - 2 - 1);
  bit = OW_GET_PIN_STATE();

  /* The complete read cycle must last at least 60 us. We have now spent */
  /* about 14-15 us in delays, so add another delay to reach >= 60 us */
  clock_delay_usec(50);

  /* Release bus */
  OW_SET_PIN_HIGH();
  OW_SET_INPUT();

  /* Allow > 1 us between read/write operations */
  clock_delay_usec(2);

  return bit ? 1 : 0;
}
/*  */
/* Read one byte of information. A byte is read least significant bit first */
/*  */

uint8_t
read_byte(void)
{
  uint8_t result = 0;
  uint8_t bit;
  int i;

  for(i = 0; i < 8; i++) {
    bit = read_bit();
    result += (bit << i);
  }
  return result;
}
/*  */
/* Write one byte of information. A byte is written least significant bit first */
/*  */

void
write_byte(uint8_t byte)
{
  int i;

  for(i = 0; i < 8; i++) {
    write_bit((byte >> i) & 1);
  }
}
/*  */
/* ds18b20_get_temp returns the temperature in "temp" (in degrees celsius) */
/* Returns 0 on failure (and then "temp" is left unchanged */
/* Returns 1 on success, and sets temp */
/*  */

uint8_t
ds18b20_get_temp(float *temp)
{
  uint8_t result = 0;

  /* Reset bus by probing. Probe returns 1 on success/presence of sensor */
  if(ds18b20_probe()) {
    /* write command "skip rom" since we only have one sensor on the wire! */
    write_byte(DS18B20_COMMAND_SKIP_ROM);

    /* write command to start measurement */
    write_byte(DS18B20_COMMAND_START_CONVERSION);

    /* Wait for conversion to complete */
    /* Conversion is 12-bit by default. */
    /* Since we have external power to the sensor (ie not in "parasitic power" mode) */
    /* the bus is held LOW by the sensor while the conversion is going on, and then HIGH */
    /* when conversion is finished. */
    OW_SET_INPUT();
    int count = 0;
    while(!OW_GET_PIN_STATE()) {
      clock_delay_msec(10);
      count++;
      /* Longest conversion time is 750 ms (12-bit resolution) */
      /* So if count > 80 (for a little margin!), we return -274.0 */
      /* which indicates failure to read the temperature. */
      if(count > 80) {
        return 0;
      }
    }

    /* The result is stored in the "scratch pad", a 9 byte memory block. */
    /* The first two bytes are the conversion result. Reading the scratch pad */
    /* can be terminated by sending a reset signal (but we read all 9 bytes) */
    (void)ds18b20_probe();
    write_byte(DS18B20_COMMAND_SKIP_ROM);
    write_byte(DS18B20_COMMAND_READ_SCRATCH_PAD);
    uint8_t i, sp_arr[9];
    for(i = 0; i < 9; i++) {
      sp_arr[i] = read_byte();
    }

    /* Check CRC, if mismatch, return 0 (failure to read temperature) */
    uint8_t crc_cal = crc8_ds18b20(sp_arr, 8);

    if(crc_cal != sp_arr[8]) {
      return 0;
    }

    /* OK, now decode what the temperature reading is. This code assumes 12-bit resolution, */
    /* so this must be modified if the code is modified to use any other resolution! */
    int16_t temp_res;
    uint8_t temp_lsb = sp_arr[0];
    uint8_t temp_msb = sp_arr[1];

    temp_res = (int16_t)temp_msb << 8 | temp_lsb;
    *temp = (float)temp_res * 0.0625;

    result = 1;
  }
  return result;
}
/*  */
/* crc8 algorithm for ds18b20 */
/* http://www.miscel.dk/MiscEl/CRCcalculations.html */
/*  */

uint8_t
crc8_ds18b20(uint8_t *buf, uint8_t buf_len)
{
  uint8_t result = 0;
  uint8_t i, b;

  for(i = 0; i < buf_len; i++) {
    result = result ^ buf[i];
    for(b = 1; b < 9; b++) {
      if(result & 0x1) {
        result = (result >> 1) ^ 0x8C;
      } else {
        result = result >> 1;
      }
    }
  }
  return result;
}