temp_pre = temp_pre >> 2; // Drop 2 lowest bits

        temp_frac = temp_pre & 0b11; // get fractional part

        temp_frac *= 25; // each bit is .25 a degree, up to fixed point

        temp_pre = temp_pre >> 2; // Drop 2 fractional bits 

        if(sign) {

            temp = -temp_pre;

        }

        else {

            temp = temp_pre;

        }

    }

    // Deassert CS

    delay(1);

    GPIO_SetBits(MAX_CS);

}

// 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

  // Calculate integral term with 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;

}

uint32_t ticks = 0;

uint32_t last_ssr_on = 0;

uint32_t last_led = 0;

int32_t setpoint = 0;

uint16_t k_p = 1;

uint16_t k_i = 1;

uint16_t k_d = 1;

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

uint8_t pid_enabled = 0;

// Process things

void process()

{

    update_temp(); // Read MAX31855

    // TODO: Add calibration offset (linear)

    if(ticks - last_led > 400) 

    {

        GPIO_ToggleBits(LED_POWER);

        last_led = ticks;

    }

    // Every 200ms, set the SSR on unless output is 0

    if((ticks - last_ssr_on > SSR_PERIOD))

    {

        if(pid_enabled) 

        {

            // 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