diff --git a/board/board.h b/board/board.h
index f89c601..9e250d8 100644
--- a/board/board.h
+++ b/board/board.h
@@ -9,6 +9,7 @@
 #ifdef PANDA
   #include "drivers/fan.h"
   #include "drivers/rtc.h"
+  #include "drivers/clock_source.h"
   #include "boards/white.h"
   #include "boards/grey.h"
   #include "boards/black.h"
diff --git a/board/board_declarations.h b/board/board_declarations.h
index d5e9e06..b6f7772 100644
--- a/board/board_declarations.h
+++ b/board/board_declarations.h
@@ -12,6 +12,7 @@ typedef uint32_t (*board_read_current)(void);
 typedef void (*board_set_ir_power)(uint8_t percentage);
 typedef void (*board_set_fan_power)(uint8_t percentage);
 typedef void (*board_set_phone_power)(bool enabled);
+typedef void (*board_set_clock_source_mode)(uint8_t mode);
 
 struct board {
   const char *board_type;
@@ -29,6 +30,7 @@ struct board {
   board_set_ir_power set_ir_power;
   board_set_fan_power set_fan_power;
   board_set_phone_power set_phone_power;
+  board_set_clock_source_mode set_clock_source_mode;
 };
 
 // ******************* Definitions ********************
diff --git a/board/boards/black.h b/board/boards/black.h
index 5730598..70248c7 100644
--- a/board/boards/black.h
+++ b/board/boards/black.h
@@ -154,6 +154,10 @@ void black_set_phone_power(bool enabled){
   UNUSED(enabled);
 }
 
+void black_set_clock_source_mode(uint8_t mode){
+  UNUSED(mode);
+}
+
 void black_init(void) {
   common_init_gpio();
 
@@ -239,5 +243,6 @@ const board board_black = {
   .read_current = black_read_current,
   .set_fan_power = black_set_fan_power,
   .set_ir_power = black_set_ir_power,
-  .set_phone_power = black_set_phone_power
+  .set_phone_power = black_set_phone_power,
+  .set_clock_source_mode = black_set_clock_source_mode
 };
diff --git a/board/boards/common.h b/board/boards/common.h
index 7343809..040ae39 100644
--- a/board/boards/common.h
+++ b/board/boards/common.h
@@ -65,7 +65,7 @@ void peripherals_init(void){
   RCC->APB1ENR |= RCC_APB1ENR_PWREN;   // for RTC config
   RCC->APB2ENR |= RCC_APB2ENR_USART1EN;
   RCC->AHB2ENR |= RCC_AHB2ENR_OTGFSEN;
-  //RCC->APB2ENR |= RCC_APB2ENR_TIM1EN;
+  RCC->APB2ENR |= RCC_APB2ENR_TIM1EN;  // clock source timer
   RCC->APB2ENR |= RCC_APB2ENR_ADC1EN;
   RCC->APB2ENR |= RCC_APB2ENR_SPI1EN;
   RCC->APB2ENR |= RCC_APB2ENR_SYSCFGEN;
diff --git a/board/boards/dos.h b/board/boards/dos.h
index afccd37..91bb313 100644
--- a/board/boards/dos.h
+++ b/board/boards/dos.h
@@ -132,6 +132,10 @@ uint32_t dos_read_current(void){
   return 0U;
 }
 
+void dos_set_clock_source_mode(uint8_t mode){
+  clock_source_init(mode);
+}
+
 void dos_init(void) {
   common_init_gpio();
 
@@ -189,6 +193,9 @@ void dos_init(void) {
 
   // init multiplexer
   can_set_obd(car_harness_status, false);
+
+  // Init clock source as internal free running
+  dos_set_clock_source_mode(CLOCK_SOURCE_MODE_FREE_RUNNING);
 }
 
 const harness_configuration dos_harness_config = {
@@ -220,5 +227,6 @@ const board board_dos = {
   .read_current = dos_read_current,
   .set_fan_power = dos_set_fan_power,
   .set_ir_power = dos_set_ir_power,
-  .set_phone_power = dos_set_phone_power
+  .set_phone_power = dos_set_phone_power,
+  .set_clock_source_mode = dos_set_clock_source_mode
 };
diff --git a/board/boards/grey.h b/board/boards/grey.h
index 1c7278d..1a7668d 100644
--- a/board/boards/grey.h
+++ b/board/boards/grey.h
@@ -48,5 +48,6 @@ const board board_grey = {
   .read_current = white_read_current,
   .set_fan_power = white_set_fan_power,
   .set_ir_power = white_set_ir_power,
-  .set_phone_power = white_set_phone_power
+  .set_phone_power = white_set_phone_power,
+  .set_clock_source_mode = white_set_clock_source_mode
 };
\ No newline at end of file
diff --git a/board/boards/pedal.h b/board/boards/pedal.h
index c67d391..b3d00f5 100644
--- a/board/boards/pedal.h
+++ b/board/boards/pedal.h
@@ -77,6 +77,10 @@ void pedal_set_phone_power(bool enabled){
   UNUSED(enabled);
 }
 
+void pedal_set_clock_source_mode(uint8_t mode){
+  UNUSED(mode);
+}
+
 void pedal_init(void) {
   common_init_gpio();
 
@@ -113,5 +117,6 @@ const board board_pedal = {
   .read_current = pedal_read_current,
   .set_fan_power = pedal_set_fan_power,
   .set_ir_power = pedal_set_ir_power,
-  .set_phone_power = pedal_set_phone_power
+  .set_phone_power = pedal_set_phone_power,
+  .set_clock_source_mode = pedal_set_clock_source_mode
 };
\ No newline at end of file
diff --git a/board/boards/uno.h b/board/boards/uno.h
index de430cb..aa5b06f 100644
--- a/board/boards/uno.h
+++ b/board/boards/uno.h
@@ -175,6 +175,10 @@ uint32_t uno_read_current(void){
   return 0U;
 }
 
+void uno_set_clock_source_mode(uint8_t mode){
+  UNUSED(mode);
+}
+
 void uno_init(void) {
   common_init_gpio();
 
@@ -282,5 +286,6 @@ const board board_uno = {
   .read_current = uno_read_current,
   .set_fan_power = uno_set_fan_power,
   .set_ir_power = uno_set_ir_power,
-  .set_phone_power = uno_set_phone_power
+  .set_phone_power = uno_set_phone_power,
+  .set_clock_source_mode = uno_set_clock_source_mode
 };
diff --git a/board/boards/white.h b/board/boards/white.h
index 1be0702..b6e639f 100644
--- a/board/boards/white.h
+++ b/board/boards/white.h
@@ -242,6 +242,10 @@ void white_set_phone_power(bool enabled){
   UNUSED(enabled);
 }
 
+void white_set_clock_source_mode(uint8_t mode){
+  UNUSED(mode);
+}
+
 void white_grey_common_init(void) {
   common_init_gpio();
 
@@ -343,5 +347,6 @@ const board board_white = {
   .read_current = white_read_current,
   .set_fan_power = white_set_fan_power,
   .set_ir_power = white_set_ir_power,
-  .set_phone_power = white_set_phone_power
+  .set_phone_power = white_set_phone_power,
+  .set_clock_source_mode = white_set_clock_source_mode
 };
diff --git a/board/drivers/clock_source.h b/board/drivers/clock_source.h
new file mode 100644
index 0000000..966dee4
--- /dev/null
+++ b/board/drivers/clock_source.h
@@ -0,0 +1,100 @@
+
+#define CLOCK_SOURCE_MODE_DISABLED       0U
+#define CLOCK_SOURCE_MODE_FREE_RUNNING   1U
+#define CLOCK_SOURCE_MODE_EXTERNAL_SYNC  2U
+
+#define CLOCK_SOURCE_PERIOD_MS           50U
+#define CLOCK_SOURCE_PULSE_LEN_MS        2U
+
+uint8_t clock_source_mode = CLOCK_SOURCE_MODE_DISABLED;
+
+void EXTI0_IRQ_Handler(void) {
+  volatile unsigned int pr = EXTI->PR & (1U << 0);
+  if (pr != 0U) {
+    if(clock_source_mode == CLOCK_SOURCE_MODE_EXTERNAL_SYNC){
+      // TODO: Implement!
+    }
+  }
+  EXTI->PR = (1U << 0);
+}
+
+void TIM1_UP_TIM10_IRQ_Handler(void) {
+  if((TIM1->SR & TIM_SR_UIF) != 0) {
+    if(clock_source_mode != CLOCK_SOURCE_MODE_DISABLED) {
+      // Start clock pulse
+      set_gpio_output(GPIOB, 14, true);
+      set_gpio_output(GPIOB, 15, true);
+    }
+
+    // Reset interrupt
+    TIM1->SR &= ~(TIM_SR_UIF);
+  }
+}
+
+void TIM1_CC_IRQ_Handler(void) {
+  if((TIM1->SR & TIM_SR_CC1IF) != 0) {
+    if(clock_source_mode != CLOCK_SOURCE_MODE_DISABLED) {
+      // End clock pulse
+      set_gpio_output(GPIOB, 14, false);
+      set_gpio_output(GPIOB, 15, false);
+    }
+
+    // Reset interrupt
+    TIM1->SR &= ~(TIM_SR_CC1IF);
+  }
+}
+
+void clock_source_init(uint8_t mode){
+  // Setup external clock signal interrupt
+  REGISTER_INTERRUPT(EXTI0_IRQn, EXTI0_IRQ_Handler, 110U, FAULT_INTERRUPT_RATE_CLOCK_SOURCE)
+  register_set(&(SYSCFG->EXTICR[0]), SYSCFG_EXTICR1_EXTI0_PB, 0xFU);
+  register_set_bits(&(EXTI->IMR), (1U << 0));
+  register_set_bits(&(EXTI->RTSR), (1U << 0));
+  register_clear_bits(&(EXTI->FTSR), (1U << 0));
+
+  // Setup timer
+  REGISTER_INTERRUPT(TIM1_UP_TIM10_IRQn, TIM1_UP_TIM10_IRQ_Handler, (1200U / CLOCK_SOURCE_PERIOD_MS) , FAULT_INTERRUPT_RATE_TIM1)
+  REGISTER_INTERRUPT(TIM1_CC_IRQn, TIM1_CC_IRQ_Handler, (1200U / CLOCK_SOURCE_PERIOD_MS) , FAULT_INTERRUPT_RATE_TIM1)
+  register_set(&(TIM1->PSC), (9600-1), 0xFFFFU);                              // Tick on 0.1 ms
+  register_set(&(TIM1->ARR), ((CLOCK_SOURCE_PERIOD_MS*10U) - 1U), 0xFFFFU);   // Period
+  register_set(&(TIM1->CCMR1), 0U, 0xFFFFU);                                  // No output on compare
+  register_set_bits(&(TIM1->CCER), TIM_CCER_CC1E);                            // Enable compare 1
+  register_set(&(TIM1->CCR1), (CLOCK_SOURCE_PULSE_LEN_MS*10U), 0xFFFFU);      // Compare 1 value
+  register_set_bits(&(TIM1->DIER), TIM_DIER_UIE | TIM_DIER_CC1IE);            // Enable interrupts
+  register_set(&(TIM1->CR1), TIM_CR1_CEN, 0x3FU);                             // Enable timer
+
+  // Set mode
+  switch(mode) {
+    case CLOCK_SOURCE_MODE_DISABLED:
+      // No clock signal
+      NVIC_DisableIRQ(EXTI0_IRQn);
+      NVIC_DisableIRQ(TIM1_UP_TIM10_IRQn);
+      NVIC_DisableIRQ(TIM1_CC_IRQn);
+
+      // Disable pulse if we were in the middle of it
+      set_gpio_output(GPIOB, 14, false);
+      set_gpio_output(GPIOB, 15, false);
+
+      clock_source_mode = CLOCK_SOURCE_MODE_DISABLED;
+      break;
+    case CLOCK_SOURCE_MODE_FREE_RUNNING:
+      // Clock signal is based on internal timer
+      NVIC_DisableIRQ(EXTI0_IRQn);
+      NVIC_EnableIRQ(TIM1_UP_TIM10_IRQn);
+      NVIC_EnableIRQ(TIM1_CC_IRQn);
+
+      clock_source_mode = CLOCK_SOURCE_MODE_FREE_RUNNING;
+      break;
+    case CLOCK_SOURCE_MODE_EXTERNAL_SYNC:
+      // Clock signal is based on external timer
+      NVIC_EnableIRQ(EXTI0_IRQn);
+      NVIC_EnableIRQ(TIM1_UP_TIM10_IRQn);
+      NVIC_EnableIRQ(TIM1_CC_IRQn);
+
+      clock_source_mode = CLOCK_SOURCE_MODE_EXTERNAL_SYNC;
+      break;
+    default:
+      puts("Unknown clock source mode: "); puth(mode); puts("\n");
+      break;
+  }
+}
\ No newline at end of file
diff --git a/board/faults.h b/board/faults.h
index 2c031cf..0825586 100644
--- a/board/faults.h
+++ b/board/faults.h
@@ -3,26 +3,28 @@
 #define FAULT_STATUS_PERMANENT 2U
 
 // Fault types
-#define FAULT_RELAY_MALFUNCTION         (1U << 0)
-#define FAULT_UNUSED_INTERRUPT_HANDLED  (1U << 1)
-#define FAULT_INTERRUPT_RATE_CAN_1      (1U << 2)
-#define FAULT_INTERRUPT_RATE_CAN_2      (1U << 3)
-#define FAULT_INTERRUPT_RATE_CAN_3      (1U << 4)
-#define FAULT_INTERRUPT_RATE_TACH       (1U << 5)
-#define FAULT_INTERRUPT_RATE_GMLAN      (1U << 6)
-#define FAULT_INTERRUPT_RATE_INTERRUPTS (1U << 7)
-#define FAULT_INTERRUPT_RATE_SPI_DMA    (1U << 8)
-#define FAULT_INTERRUPT_RATE_SPI_CS     (1U << 9)
-#define FAULT_INTERRUPT_RATE_UART_1     (1U << 10)
-#define FAULT_INTERRUPT_RATE_UART_2     (1U << 11)
-#define FAULT_INTERRUPT_RATE_UART_3     (1U << 12)
-#define FAULT_INTERRUPT_RATE_UART_5     (1U << 13)
-#define FAULT_INTERRUPT_RATE_UART_DMA   (1U << 14)
-#define FAULT_INTERRUPT_RATE_USB        (1U << 15)
-#define FAULT_INTERRUPT_RATE_TIM1       (1U << 16)
-#define FAULT_INTERRUPT_RATE_TIM3       (1U << 17)
-#define FAULT_REGISTER_DIVERGENT        (1U << 18)
-#define FAULT_INTERRUPT_RATE_KLINE_INIT (1U << 19)
+#define FAULT_RELAY_MALFUNCTION             (1U << 0)
+#define FAULT_UNUSED_INTERRUPT_HANDLED      (1U << 1)
+#define FAULT_INTERRUPT_RATE_CAN_1          (1U << 2)
+#define FAULT_INTERRUPT_RATE_CAN_2          (1U << 3)
+#define FAULT_INTERRUPT_RATE_CAN_3          (1U << 4)
+#define FAULT_INTERRUPT_RATE_TACH           (1U << 5)
+#define FAULT_INTERRUPT_RATE_GMLAN          (1U << 6)
+#define FAULT_INTERRUPT_RATE_INTERRUPTS     (1U << 7)
+#define FAULT_INTERRUPT_RATE_SPI_DMA        (1U << 8)
+#define FAULT_INTERRUPT_RATE_SPI_CS         (1U << 9)
+#define FAULT_INTERRUPT_RATE_UART_1         (1U << 10)
+#define FAULT_INTERRUPT_RATE_UART_2         (1U << 11)
+#define FAULT_INTERRUPT_RATE_UART_3         (1U << 12)
+#define FAULT_INTERRUPT_RATE_UART_5         (1U << 13)
+#define FAULT_INTERRUPT_RATE_UART_DMA       (1U << 14)
+#define FAULT_INTERRUPT_RATE_USB            (1U << 15)
+#define FAULT_INTERRUPT_RATE_TIM1           (1U << 16)
+#define FAULT_INTERRUPT_RATE_TIM3           (1U << 17)
+#define FAULT_REGISTER_DIVERGENT            (1U << 18)
+#define FAULT_INTERRUPT_RATE_KLINE_INIT     (1U << 19)
+#define FAULT_INTERRUPT_RATE_CLOCK_SOURCE   (1U << 20)
+#define FAULT_INTERRUPT_RATE_TIM9           (1U << 21)
 
 // Permanent faults
 #define PERMANENT_FAULTS 0U
diff --git a/board/main.c b/board/main.c
index 497ea5d..207e5dc 100644
--- a/board/main.c
+++ b/board/main.c
@@ -606,6 +606,10 @@ int usb_cb_control_msg(USB_Setup_TypeDef *setup, uint8_t *resp, bool hardwired)
         }
       }
       break;
+    // **** 0xf5: set clock source mode
+    case 0xf5:
+      current_board->set_clock_source_mode(setup->b.wValue.w);
+      break;
     default:
       puts("NO HANDLER ");
       puth(setup->b.bRequest);
@@ -754,7 +758,7 @@ int main(void) {
   init_interrupts(true);
 
   // 1s timer
-  REGISTER_INTERRUPT(TIM1_BRK_TIM9_IRQn, TIM1_BRK_TIM9_IRQ_Handler, 2U, FAULT_INTERRUPT_RATE_TIM1)
+  REGISTER_INTERRUPT(TIM1_BRK_TIM9_IRQn, TIM1_BRK_TIM9_IRQ_Handler, 2U, FAULT_INTERRUPT_RATE_TIM9)
 
   // shouldn't have interrupts here, but just in case
   disable_interrupts();
diff --git a/python/__init__.py b/python/__init__.py
index 605977d..c602305 100644
--- a/python/__init__.py
+++ b/python/__init__.py
@@ -148,6 +148,10 @@ class Panda(object):
   HW_TYPE_PEDAL = b'\x04'
   HW_TYPE_UNO = b'\x05'
 
+  CLOCK_SOURCE_MODE_DISABLED = 0
+  CLOCK_SOURCE_MODE_FREE_RUNNING = 1
+  CLOCK_SOURCE_MODE_EXTERNAL_SYNC = 2
+
   def __init__(self, serial=None, claim=True):
     self._serial = serial
     self._handle = None
@@ -665,3 +669,7 @@ class Panda(object):
   # ****************** Phone *****************
   def set_phone_power(self, enabled):
     self._handle.controlWrite(Panda.REQUEST_OUT, 0xb3, int(enabled), 0, b'')
+
+  # ************** Clock Source **************
+  def set_clock_source_mode(self, mode):
+    self._handle.controlWrite(Panda.REQUEST_OUT, 0xf5, int(mode), 0, b'')