dht22.c

#include "dht22.h"
#include "tim.h"

#include <stm32f10x_rcc.h>
#include <stm32f10x_gpio.h>

static GPIO_InitTypeDef PORT;

void
DHT22_Init (void)
{
  RCC_APB2PeriphClockCmd (DHT22_GPIO_CLOCK, ENABLE);
  PORT.GPIO_Pin = DHT22_GPIO_PIN;
  PORT.GPIO_Speed = GPIO_Speed_50MHz;
}

static uint8_t
DHT22_GetReadings (dht22_data *out)
{
  uint8_t i;
  uint16_t c;

  // Switch pin to output
  PORT.GPIO_Mode = GPIO_Mode_Out_PP;
  GPIO_Init (DHT22_GPIO_PORT, &PORT);

  // Generate start impulse
  DHT22_GPIO_PORT->BRR = DHT22_GPIO_PIN; // Pull down SDA (Bit_SET)
  Delay_ms (2); // Host start signal at least 1ms
  DHT22_GPIO_PORT->BSRR = DHT22_GPIO_PIN; // Release SDA (Bit_RESET)

  // Switch pin to input with Pull-Up
  PORT.GPIO_Mode = GPIO_Mode_IPU;
  GPIO_Init (DHT22_GPIO_PORT, &PORT);

  // Wait for AM2302 to begin communication (20-40us)
  DELAY_US_TIM->CNT = 0;
  while (((c = DELAY_US_TIM->CNT) < 100)
      && (DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
    ;

  DELAY_US_TIM->CNT = 0;

  if (c >= 100)
    return DHT22_RCV_NO_RESPONSE;

  // Check ACK strobe from sensor
  while (((c = DELAY_US_TIM->CNT) < 100)
      && !(DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
    ;

  DELAY_US_TIM->CNT = 0;

  if ((c < 65) || (c > 95))
    return DHT22_RCV_BAD_ACK1;

  while (((c = DELAY_US_TIM->CNT) < 100)
      && (DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
    ;

  DELAY_US_TIM->CNT = 0;

  if ((c < 65) || (c > 95))
    return DHT22_RCV_BAD_ACK2;

  // ACK strobe received --> receive 40 bits
  i = 0;

  while (i < 40)
    {
      // Measure bit start impulse (T_low = 50us)

      while (((c = DELAY_US_TIM->CNT) < 100)
	  && !(DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
	;

      if (c > 75)
	{
	  // invalid bit start impulse length
	  out->bits[i] = 0xff;
	  while (((c = DELAY_US_TIM->CNT) < 100)
	      && (DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
	    ;
	  DELAY_US_TIM->CNT = 0;
	}
      else
	{
	  // Measure bit impulse length (T_h0 = 25us, T_h1 = 70us)
	  DELAY_US_TIM->CNT = 0;
	  while (((c = DELAY_US_TIM->CNT) < 100)
	      && (DHT22_GPIO_PORT->IDR & DHT22_GPIO_PIN))
	    ;
	  DELAY_US_TIM->CNT = 0;
	  out->bits[i] = (c < 100) ? (uint8_t) c : 0xff;
	}

      i++;
    }

  for (i = 0; i < 40; i++)
    if (out->bits[i] == 0xff)
      return DHT22_RCV_TIMEOUT;

  return DHT22_RCV_OK;
}

static void
DHT22_DecodeReadings (dht22_data *out)
{
  uint8_t i = 0;

  for (; i < 8; i++)
    {
      out->hMSB <<= 1;
      if (out->bits[i] > 48)
	out->hMSB |= 1;
    }

  for (; i < 16; i++)
    {
      out->hLSB <<= 1;
      if (out->bits[i] > 48)
	out->hLSB |= 1;
    }

  for (; i < 24; i++)
    {
      out->tMSB <<= 1;
      if (out->bits[i] > 48)
	out->tMSB |= 1;
    }

  for (; i < 32; i++)
    {
      out->tLSB <<= 1;
      if (out->bits[i] > 48)
	out->tLSB |= 1;
    }

  for (; i < 40; i++)
    {
      out->parity_rcv <<= 1;
      if (out->bits[i] > 48)
	out->parity_rcv |= 1;
    }

  out->parity = out->hMSB + out->hLSB + out->tMSB + out->tLSB;

}

static uint16_t
DHT22_GetHumidity (dht22_data *out)
{
  return (out->hMSB << 8) | out->hLSB;
}

static uint16_t
DHT22_GetTemperature (dht22_data *out)
{
  return (out->tMSB << 8) | out->tLSB;
}

bool
DHT22_Read (dht22_data *out)
{
  out->rcv_response = DHT22_GetReadings (out);
  if (out->rcv_response != DHT22_RCV_OK)
    {
      return false;
    }

  DHT22_DecodeReadings (out);

  if (out->parity != out->parity_rcv)
    {
      out->rcv_response = DHT22_BAD_DATA;
      return false;
    }

  out->humidity = (float) DHT22_GetHumidity (out) / 10.0f;

  uint16_t temperature = DHT22_GetTemperature (out);
  out->temperature = ((float) (temperature & 0x7fff)) / 10.0f;

  if (temperature & 0x8000)
    {
      out->temperature = -out->temperature;
    }

  return true;

}