GPIO_ResetBits(LED_STAT);

// PID implementation

int16_t last_pid_temp = 0;

uint8_t last_pid_temp_frac = 0;

int16_t i_state = 0;

#define WINDUP_GUARD_GAIN 100

int16_t update_pid(uint16_t k_p, uint16_t k_i, uint16_t k_d, int16_t temp, uint8_t temp_frac, int16_t setpoint) 

{

  // Calculate instantaneous error

  int16_t error = (int16_t)setpoint - (int16_t)temp; // TODO: Use fixed point fraction

  // Proportional component

  int16_t p_term = k_p * error;

  // Error accumulator (integrator)

  i_state += error;

  // to prevent the iTerm getting huge despite lots of 

  //  error, we use a "windup guard" 

  // (this happens when the machine is first turned on and

  // it cant help be cold despite its best efforts)

  // not necessary, but this makes windup guard values 

  // relative to the current iGain

  int16_t windup_guard = WINDUP_GUARD_GAIN / k_i;  

  // Calculate integral term with windup guard 

  if (i_state > windup_guard) 

    i_state = windup_guard;

  else if (i_state < -windup_guard) 

    i_state = -windup_guard;

  int16_t i_term = k_i * i_state;

  // Calculate differential term (slope since last iteration)

  int16_t d_term = (k_d * (temp - last_pid_temp));

  // Save temperature for next iteration

  last_pid_temp = temp;

  last_pid_temp_frac = temp_frac;

  int16_t result = p_term + i_term - d_term;

  // Put out tenths of percent, 0-1000. 

  if(result > 1000)

    result = 1000;

  else if(result < -1000)

    result = -1000;

  // Return feedback

  return result;

}

int16_t ssr_output = 0; // Duty cycle of ssr, 0 to SSR_PERIOD 

    if((ticks - last_ssr_on > SSR_PERIOD))

        // Get ssr output for next time

        int16_t power_percent = update_pid(k_p, k_i, k_d, temp, temp_frac, setpoint);

        //power-percent is 0-1000

        ssr_output = power_percent; //(((uint32_t)SSR_PERIOD * (uint32_t)10 * (uint32_t)100) * power_percent) / (uint32_t)1000000;

        // Only support heating (ssr_output > 0) right now

        if(ssr_output > 0) {

            char tempstr[6];

            itoa(ssr_output, tempstr);

            ssd1306_DrawString("#=", 2, 45);

            ssd1306_DrawString("    ", 2, 57);

            ssd1306_DrawString(tempstr, 2, 57);

            GPIO_SetBits(LED_STAT);

            last_ssr_on = ticks;

        }