libbcm2835 sensor

/*
 * Author: greg.burri@gmail.com
 * libbcm2835 documentation: http://www.airspayce.com/mikem/bcm2835/index.html
 */

#include <bcm2835.h>
#include <iostream>
#include <ctime>
using namespace std;

const auto pin = RPI_GPIO_P1_07;
const double temperatureMax = 60;
const double temperatureMin = -20;
const double humidityMax = 100;
const double humidityMin = 0;

class ReadException {};

inline uint64_t timeFrom(clock_t t)
{
   return uint64_t(clock() - t) / (CLOCKS_PER_SEC / 1000 / 1000);
}

void sync(uint64_t timeout, bool toUp)
{
   auto last = bcm2835_gpio_lev(pin);
   bool synced = false;

   clock_t t = clock();

   while (!synced && timeFrom(t) < timeout)
   {
      auto current = bcm2835_gpio_lev(pin);
      synced = toUp ? (last == LOW && current == HIGH) : (last == HIGH && current == LOW);
      last = current;
   }

   if (!synced)
   {
      // uint64_t delta = timeFrom(t);
      // cout << "Unable to sync " << (toUp ? "up" : "down") << " Time: " << delta << endl;
      throw ReadException();
   }
}

inline void syncToUp(long timeout)
{
   sync(timeout, true);
}

inline void syncToDown(long timeout)
{
   sync(timeout, false);
}

bool readNextBit()
{
   syncToUp(300); // Should be 50 us.
   clock_t t = clock();
   syncToDown(300); // Should be 70 us.
   return timeFrom(t) > 50;
}

uint8_t readNextByte()
{
   uint8_t result = 0;
   for (int i = 0; i < 8; i++)
      if (readNextBit())
         result |= (1 << (7 - i));
   return result;
}

extern "C"
bool init()
{
   if (!bcm2835_init())
   {
      cerr << "Unable to init the BCM 2835" << endl;
      return false;
   }
   return true;
}

extern "C"
void getHumidityAndTemperature(double& h, double& t)
{
   bool dataOK = false;
   while (!dataOK)
   {
      try
      {
         bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_OUTP);

         bcm2835_gpio_write(pin, LOW);
         bcm2835_delayMicroseconds(30000); // 30 ms.
         bcm2835_gpio_write(pin, HIGH);
         bcm2835_delayMicroseconds(30);

         bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_INPT);

         syncToUp(200); //  Should be 80 us.
         syncToDown(200); // Should be 80 us.

         int rh1 = readNextByte();
         int rh2 = readNextByte();
         int t1 = readNextByte();
         int t2 = readNextByte();
         int checksum = readNextByte();

         if (((rh1 + rh2 + t1 + t2) & 0xFF) != checksum)
            throw ReadException();

         h = double((rh1 << 8) | rh2) / 10.0;
         t = double(((t1 & 0x7F) << 8) | t2) / 10.0;

         if (t1 & 0x80)
            t *= -1.0;

         if (h >= humidityMin && h <= humidityMax && t >= temperatureMin && t <= temperatureMax)
         {
            dataOK = true;
         }
      }
      catch(ReadException ex)
      {
         cerr << "Read error, retrying ..." << endl;
      }

      if (!dataOK)
         bcm2835_delay(2000);
   }
}