diff --git a/src/pid.c b/src/pid.c
new file mode 100644
--- /dev/null
+++ b/src/pid.c
@@ -0,0 +1,109 @@
+//
+// PID: proportional/integral/derivative controller
+//
+
+#include "pid.h"
+#include "flash.h"
+
+// PID implementation
+
+static pid_state_t state;
+
+
+void pid_init()
+{
+	state.i_state = 0;
+	state.last_pid_temp = 0;
+	state.last_pid_temp_frac = 0;
+}
+
+float pid_process(void)
+{
+
+
+//            #ifdef MAX31865_RTD_SENSOR
+//            max31865_readtemp(spi_get(), &set, &status);
+//			#else
+//			max31855_readtemp(spi_get(), &set, &status); // Read MAX31855
+//			#endif
+
+	float ssr_output = 0;
+
+
+	if(runtime_status()->pid_enabled)
+	{
+		// Get ssr output for next time
+		int16_t power_percent = pid_update();
+
+		if(flash_getsettings()->val.plant_type == PLANT_HEATER)
+			power_percent *= -1;
+
+		//power-percent is 0-1000?
+		ssr_output = power_percent; //(((uint32_t)SSR_PERIOD * (uint32_t)10 * (uint32_t)100) * power_percent) / (uint32_t)1000000;
+
+		// put ssr output on display
+		ssd1306_drawstring("      ", 0, 90); //fixme: this is bad, but I can't get the old digits to clear otherwise
+		char tempstr[6];
+		snprintf(tempstr, 6, "%g", ssr_output);
+		ssd1306_drawstring(tempstr, 0, 90);
+	}
+	else
+	{
+		ssr_output = 0.0;
+	}
+
+	return ssr_output;
+}
+
+int16_t pid_update(void)
+{
+	therm_status_t* status = runtime_status();
+	therm_settings_t* set = flash_getsettings();
+
+	// Convert temperature to fixed point number with 1/10th resolution
+	float temp = status->temp;
+
+	// Calculate instantaneous error
+	int16_t error = status->setpoint * 10 - temp; // TODO: Use fixed point fraction
+
+	// Proportional component
+	int32_t p_term = set->val.k_p * error;
+
+	// Error accumulator (integrator)
+	state.i_state += error;
+
+	// to prevent the iTerm getting huge from 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
+	int32_t windup_guard_res = (set->val.windup_guard * 10) / set->val.k_i;
+
+	// Calculate integral term with windup guard
+	if (state.i_state > windup_guard_res)
+	  state.i_state = windup_guard_res;
+	else if (state.i_state < -windup_guard_res)
+	  state.i_state = -windup_guard_res;
+
+	int32_t i_term = set->val.k_i * state.i_state;
+
+	// Calculate differential term (slope since last iteration)
+	int32_t d_term = (set->val.k_d * (temp - state.last_pid_temp));
+
+	// Save temperature for next iteration
+	state.last_pid_temp = temp;
+
+	int16_t result = (p_term + i_term - d_term) / 10;
+
+	// Put out tenths of percent, 0-1000.
+	if(result > 1000)
+	result = 1000;
+	else if(result < -1000)
+	result = -1000;
+
+	// Return feedback
+	return result;
+}
+
+