diff --git a/main.c b/main.c
--- a/main.c
+++ b/main.c
@@ -16,10 +16,6 @@
 #include "usb_device.h"
 #include "usbd_cdc_if.h"
 
-
-// Prototypes
-void process();
-
 therm_settings_t set;
 therm_status_t status;
 
@@ -32,7 +28,8 @@ int main(void)
     systemclock_init();
 
     // Unset bootloader option bytes (if set)
-    void bootloader_unset(void);
+    // FIXME this was never getting called. Try again sometime.
+    //bootloader_unset();
 
     // Init GPIO
     gpio_init();
@@ -42,7 +39,7 @@ int main(void)
 //    set.val.usb_plugged = 
 
     // USB startup delay
-    HAL_Delay(1000);
+    HAL_Delay(500);
     HAL_GPIO_WritePin(LED_POWER, 1);
 
     // Enter into bootloader if up button pressed on boot
@@ -86,11 +83,11 @@ int main(void)
     ssd1306_DrawString("therm v0.2", 1, 40);
     ssd1306_DrawString("protofusion.org/therm", 3, 0);
 
-    HAL_Delay(1500);
+    HAL_Delay(1000);
 
     flash_restore(&set);
 
-    HAL_Delay(1500);
+    HAL_Delay(1000);
     ssd1306_clearscreen();
 
     // Soft timers
diff --git a/pid.c b/pid.c
new file mode 100644
--- /dev/null
+++ b/pid.c
@@ -0,0 +1,56 @@
+#include "stm32f0xx_hal.h"
+#include "states.h"
+
+// PID implementation
+// TODO: Make struct that has the last_temp and i_state in it, pass by ref. Make struct that has other input values maybe.
+static int16_t last_pid_temp = 0;
+static uint8_t last_pid_temp_frac = 0;
+static int32_t i_state = 0;
+
+int16_t pid_update(uint16_t k_p, uint16_t k_i, uint16_t k_d, int16_t temp, uint8_t temp_frac, int16_t setpoint, therm_settings_t* set, therm_status_t* status) 
+{
+  // Calculate instantaneous error
+  int16_t error = setpoint - temp; // TODO: Use fixed point fraction
+
+  // Proportional component
+  int32_t p_term = k_p * error;
+
+  // Error accumulator (integrator)
+  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 / k_i;  
+
+  // Calculate integral term with windup guard 
+  if (i_state > windup_guard_res) 
+    i_state = windup_guard_res;
+  else if (i_state < -windup_guard_res) 
+    i_state = -windup_guard_res;
+
+  int32_t i_term = k_i * i_state;
+
+  // Calculate differential term (slope since last iteration)
+  int32_t d_term = (k_d * (status->temp - last_pid_temp));
+
+  // Save temperature for next iteration
+  last_pid_temp = status->temp;
+  last_pid_temp_frac = status->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;
+}
+
+
diff --git a/pid.h b/pid.h
new file mode 100644
--- /dev/null
+++ b/pid.h
@@ -0,0 +1,8 @@
+#ifndef PIDS_H
+#define PIDS_H
+
+#include "states.h"
+
+int16_t pid_update(uint16_t k_p, uint16_t k_i, uint16_t k_d, int16_t temp, uint8_t temp_frac, int16_t setpoint, therm_settings_t* set, therm_status_t* status);
+
+#endif