diff --git a/Core/Inc/AMS_CAN.h b/Core/Inc/AMS_CAN.h
index 4e42aaf..5a20b43 100644
--- a/Core/Inc/AMS_CAN.h
+++ b/Core/Inc/AMS_CAN.h
@@ -17,6 +17,7 @@
 #include <stdint.h>
 
 #define CAN_ID_SLAVE_ERROR 0x001
+#define CAN_ID_TIME_SYNC 0x002
 #define CAN_ID_AMS_SLAVE_HEARTBEAT_BASE 0x600
 #define CAN_HEARTBEAT_TX_TIMEOUT 10 /* ms */
 
diff --git a/Core/Inc/TimeSync.h b/Core/Inc/TimeSync.h
new file mode 100644
index 0000000..842c7ee
--- /dev/null
+++ b/Core/Inc/TimeSync.h
@@ -0,0 +1,13 @@
+#ifndef INC_TIME_SYNC_H_
+#define INC_TIME_SYNC_H_
+
+#define TARGET_FREQ 16000000    // Hz
+#define TRIM_FREQ 48000         // Hz
+#define TIME_SYNC_INTERVAL 1000 // ms
+#define RCC_CR_HSITRIM_MAX 31
+
+#define STARTUP_TIME_SYNC_TIME 5000 // ms
+
+void time_sync_handle_frame();
+
+#endif // INC_TIME_SYNC_H_
diff --git a/Core/Src/AMS_CAN.c b/Core/Src/AMS_CAN.c
index 1b94e20..c17d693 100644
--- a/Core/Src/AMS_CAN.c
+++ b/Core/Src/AMS_CAN.c
@@ -9,6 +9,7 @@
 
 #include "BQ_Abstraction_Layer.h"
 #include "TMP144.h"
+#include "TimeSync.h"
 #include "common_defs.h"
 #include "main.h"
 
@@ -32,6 +33,30 @@ void ams_can_init(CAN_HandleTypeDef* ams_handle,
     }
   }
 
+  // Config filter
+  CAN_FilterTypeDef can_filter;
+  can_filter.FilterActivation = CAN_FILTER_ENABLE;
+  can_filter.FilterBank = 0;
+  can_filter.FilterFIFOAssignment = CAN_FILTER_FIFO0;
+  /* Message ID is in the MSBs of the FilterId register */
+  can_filter.FilterIdHigh = CAN_ID_TIME_SYNC << (16 - 11);
+  can_filter.FilterIdLow = 0;
+  /* Filter the 11 MSBs (i.e. a StdId) */
+  can_filter.FilterMaskIdHigh = 0xFFE0;
+  can_filter.FilterMaskIdLow = 0;
+  can_filter.FilterMode = CAN_FILTERMODE_IDMASK;
+  can_filter.FilterScale = CAN_FILTERSCALE_32BIT;
+  // If we use CAN1, the slave filter should start after our filter bank. If we
+  // use CAN2, it should start at our filter bank.
+  if (handle_ams == ams_handle) {
+    can_filter.SlaveStartFilterBank = 1;
+  } else {
+    can_filter.SlaveStartFilterBank = 0;
+  }
+  if (HAL_CAN_ConfigFilter(handle_ams, &can_filter) != HAL_OK) {
+    Error_Handler();
+  }
+
   // Activate RX notifications
   if (HAL_CAN_ActivateNotification(handle_ams, CAN_IT_RX_FIFO0_MSG_PENDING) !=
       HAL_OK) {
@@ -57,7 +82,17 @@ void HAL_CAN_RxFifo0MsgPendingCallback(CAN_HandleTypeDef* handle) {
   }
 }
 
-void ams_can_handle_ams_msg(CAN_RxHeaderTypeDef* header, uint8_t* data) {}
+void ams_can_handle_ams_msg(CAN_RxHeaderTypeDef* header, uint8_t* data) {
+  if (header->IDE != CAN_ID_STD) {
+    return;
+  }
+
+  switch (header->StdId) {
+  case CAN_ID_TIME_SYNC:
+    time_sync_handle_frame();
+    break;
+  }
+}
 
 void ams_can_send_heartbeat() {
   static CAN_TxHeaderTypeDef header;
diff --git a/Core/Src/TimeSync.c b/Core/Src/TimeSync.c
new file mode 100644
index 0000000..dd79ee9
--- /dev/null
+++ b/Core/Src/TimeSync.c
@@ -0,0 +1,53 @@
+#include "TimeSync.h"
+
+#include "stm32f412rx.h"
+#include "stm32f4xx_hal.h"
+
+#include <stdint.h>
+
+void time_sync_handle_frame() {
+  static uint32_t last_time_sync_frame = 0;
+  static uint32_t f_pre_trim = TARGET_FREQ;
+  static uint8_t trimmed_last_frame = 0;
+  static int32_t last_trim_delta = TRIM_FREQ;
+
+  uint32_t now = HAL_GetTick();
+  if (last_time_sync_frame != 0) {
+    uint32_t n_measured = now - last_time_sync_frame;
+    uint32_t f_real = n_measured * (TARGET_FREQ / TIME_SYNC_INTERVAL);
+    if (trimmed_last_frame) {
+      last_trim_delta = (f_pre_trim - f_real) / trimmed_last_frame;
+      if (last_trim_delta < 0) {
+        last_trim_delta = -last_trim_delta;
+      }
+      trimmed_last_frame = 0;
+    }
+
+    int32_t delta_f = TARGET_FREQ - f_real;
+    int32_t delta_quants = delta_f / last_trim_delta;
+    if (delta_quants != 0) {
+      uint32_t trim_delta = (now < STARTUP_TIME_SYNC_TIME) ? 2 : 1;
+      uint32_t rcc_cr = RCC->CR;
+      // Determine current trim
+      int32_t trim = (rcc_cr & RCC_CR_HSITRIM_Msk) >> RCC_CR_HSITRIM_Pos;
+      if (delta_quants > 0) {
+        trim += trim_delta;
+        if (trim > RCC_CR_HSITRIM_MAX) {
+          trim = RCC_CR_HSITRIM_MAX;
+        }
+      } else if (delta_quants < 0) {
+        trim -= trim_delta;
+        if (trim < 0) {
+          trim = 0;
+        }
+      }
+      // Clear current trim and overwrite with new trim
+      rcc_cr = (rcc_cr & ~RCC_CR_HSITRIM_Msk) |
+               ((trim << RCC_CR_HSITRIM_Pos) & RCC_CR_HSITRIM_Msk);
+      RCC->CR = rcc_cr;
+      f_pre_trim = f_real;
+      trimmed_last_frame = trim_delta;
+    }
+  }
+  last_time_sync_frame = now;
+}
diff --git a/Core/Src/main.c b/Core/Src/main.c
index c980e20..8b7fc19 100644
--- a/Core/Src/main.c
+++ b/Core/Src/main.c
@@ -26,6 +26,7 @@
 #include "EEPROM.h"
 #include "FanControl.h"
 #include "TMP144.h"
+#include "TimeSync.h"
 #include "common_defs.h"
 
 #include "stm32f4xx_hal.h"
@@ -214,7 +215,10 @@ int main(void) {
       check_error_conditions();
     }
     fan_ctrl_update();
-    ams_can_send_heartbeat();
+    // Only start sending CAN frames once the clock is somewhat synchronized
+    if (HAL_GetTick() > STARTUP_TIME_SYNC_TIME) {
+      ams_can_send_heartbeat();
+    }
     delay_period();
   }
   /* USER CODE END 3 */
@@ -236,8 +240,9 @@ void SystemClock_Config(void) {
   /** Initializes the RCC Oscillators according to the specified parameters
    * in the RCC_OscInitTypeDef structure.
    */
-  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
-  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
+  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
+  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
+  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
   RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
   if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
     Error_Handler();
@@ -247,7 +252,7 @@ void SystemClock_Config(void) {
    */
   RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_SYSCLK |
                                 RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
-  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSE;
+  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
   RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
   RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
   RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
@@ -255,10 +260,6 @@ void SystemClock_Config(void) {
   if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK) {
     Error_Handler();
   }
-
-  /** Enables the Clock Security System
-   */
-  HAL_RCC_EnableCSS();
 }
 
 /**
@@ -551,7 +552,6 @@ static void MX_GPIO_Init(void) {
   GPIO_InitTypeDef GPIO_InitStruct = {0};
 
   /* GPIO Ports Clock Enable */
-  __HAL_RCC_GPIOH_CLK_ENABLE();
   __HAL_RCC_GPIOC_CLK_ENABLE();
   __HAL_RCC_GPIOA_CLK_ENABLE();
   __HAL_RCC_GPIOB_CLK_ENABLE();
diff --git a/Core/Src/stm32f4xx_it.c b/Core/Src/stm32f4xx_it.c
index f531eec..e7b7766 100644
--- a/Core/Src/stm32f4xx_it.c
+++ b/Core/Src/stm32f4xx_it.c
@@ -77,7 +77,6 @@ void NMI_Handler(void)
   /* USER CODE BEGIN NonMaskableInt_IRQn 0 */
 
   /* USER CODE END NonMaskableInt_IRQn 0 */
-  HAL_RCC_NMI_IRQHandler();
   /* USER CODE BEGIN NonMaskableInt_IRQn 1 */
   while (1) {
   }
diff --git a/Makefile b/Makefile
index 209c29f..36e0329 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 ##########################################################################################################################
-# File automatically-generated by tool: [projectgenerator] version: [3.17.1] date: [Sat Jul 30 20:03:51 CEST 2022] 
+# File automatically-generated by tool: [projectgenerator] version: [3.17.1] date: [Sat Jul 30 20:29:35 CEST 2022] 
 ##########################################################################################################################
 
 # ------------------------------------------------
diff --git a/STM32Make.make b/STM32Make.make
index 5d6bcb7..fb2c1ed 100644
--- a/STM32Make.make
+++ b/STM32Make.make
@@ -44,6 +44,7 @@ Core/Src/EEPROM.c \
 Core/Src/FanControl.c \
 Core/Src/SoftI2C.c \
 Core/Src/TMP144.c \
+Core/Src/TimeSync.c \
 Core/Src/main.c \
 Core/Src/stm32f4xx_hal_msp.c \
 Core/Src/stm32f4xx_it.c \
diff --git a/ams-slave.ioc b/ams-slave.ioc
index 8291d56..738465f 100644
--- a/ams-slave.ioc
+++ b/ams-slave.ioc
@@ -58,35 +58,33 @@ Mcu.IP9=USART2
 Mcu.IPNb=12
 Mcu.Name=STM32F412R(E-G)Tx
 Mcu.Package=LQFP64
-Mcu.Pin0=PH0 - OSC_IN
-Mcu.Pin1=PH1 - OSC_OUT
-Mcu.Pin10=PB2
-Mcu.Pin11=PB10
-Mcu.Pin12=PB13
-Mcu.Pin13=PC6
-Mcu.Pin14=PC7
-Mcu.Pin15=PC9
-Mcu.Pin16=PA10
-Mcu.Pin17=PA11
-Mcu.Pin18=PA12
-Mcu.Pin19=PA13
-Mcu.Pin2=PC0
-Mcu.Pin20=PA14
-Mcu.Pin21=PA15
-Mcu.Pin22=PB3
-Mcu.Pin23=PB4
-Mcu.Pin24=PB5
-Mcu.Pin25=PB6
-Mcu.Pin26=PB7
-Mcu.Pin27=VP_SYS_VS_Systick
-Mcu.Pin3=PC1
-Mcu.Pin4=PC2
-Mcu.Pin5=PC3
-Mcu.Pin6=PA2
-Mcu.Pin7=PA3
-Mcu.Pin8=PC5
-Mcu.Pin9=PB0
-Mcu.PinsNb=28
+Mcu.Pin0=PC0
+Mcu.Pin1=PC1
+Mcu.Pin10=PB13
+Mcu.Pin11=PC6
+Mcu.Pin12=PC7
+Mcu.Pin13=PC9
+Mcu.Pin14=PA10
+Mcu.Pin15=PA11
+Mcu.Pin16=PA12
+Mcu.Pin17=PA13
+Mcu.Pin18=PA14
+Mcu.Pin19=PA15
+Mcu.Pin2=PC2
+Mcu.Pin20=PB3
+Mcu.Pin21=PB4
+Mcu.Pin22=PB5
+Mcu.Pin23=PB6
+Mcu.Pin24=PB7
+Mcu.Pin25=VP_SYS_VS_Systick
+Mcu.Pin3=PC3
+Mcu.Pin4=PA2
+Mcu.Pin5=PA3
+Mcu.Pin6=PC5
+Mcu.Pin7=PB0
+Mcu.Pin8=PB2
+Mcu.Pin9=PB10
+Mcu.PinsNb=26
 Mcu.ThirdPartyNb=0
 Mcu.UserConstants=
 Mcu.UserName=STM32F412RETx
@@ -105,7 +103,7 @@ NVIC.ForceEnableDMAVector=true
 NVIC.HardFault_IRQn=true\:0\:0\:false\:false\:true\:false\:false\:false
 NVIC.I2C1_EV_IRQn=true\:0\:0\:false\:false\:true\:true\:true\:true
 NVIC.MemoryManagement_IRQn=true\:0\:0\:false\:false\:true\:false\:false\:false
-NVIC.NonMaskableInt_IRQn=true\:0\:0\:false\:false\:true\:false\:true\:false
+NVIC.NonMaskableInt_IRQn=true\:0\:0\:false\:false\:true\:false\:false\:false
 NVIC.PendSV_IRQn=true\:0\:0\:false\:false\:true\:false\:false\:false
 NVIC.PriorityGroup=NVIC_PRIORITYGROUP_4
 NVIC.SVCall_IRQn=true\:0\:0\:false\:false\:true\:false\:false\:false
@@ -184,10 +182,6 @@ PC9.GPIOParameters=GPIO_Label
 PC9.GPIO_Label=FAN_PWM
 PC9.Locked=true
 PC9.Signal=S_TIM3_CH4
-PH0\ -\ OSC_IN.Mode=HSE-External-Oscillator
-PH0\ -\ OSC_IN.Signal=RCC_OSC_IN
-PH1\ -\ OSC_OUT.Mode=HSE-External-Oscillator
-PH1\ -\ OSC_OUT.Signal=RCC_OSC_OUT
 PinOutPanel.RotationAngle=0
 ProjectManager.AskForMigrate=true
 ProjectManager.BackupPrevious=false
@@ -219,10 +213,9 @@ ProjectManager.UnderRoot=false
 ProjectManager.functionlistsort=1-SystemClock_Config-RCC-false-HAL-false,2-MX_GPIO_Init-GPIO-false-HAL-true,3-MX_DMA_Init-DMA-false-HAL-true,4-MX_CAN1_Init-CAN1-false-HAL-true,5-MX_CAN2_Init-CAN2-false-HAL-true,6-MX_I2C1_Init-I2C1-false-HAL-true,7-MX_TIM3_Init-TIM3-false-HAL-true,8-MX_USART1_UART_Init-USART1-false-HAL-true,9-MX_USART2_UART_Init-USART2-false-HAL-true,10-MX_USART3_UART_Init-USART3-false-HAL-true,11-MX_USART6_UART_Init-USART6-false-HAL-true
 RCC.CortexFreq_Value=16000000
 RCC.DFSDMFreq_Value=16000000
-RCC.EnbaleCSS=true
 RCC.FamilyName=M
 RCC.HSE_VALUE=16000000
-RCC.IPParameters=CortexFreq_Value,DFSDMFreq_Value,EnbaleCSS,FamilyName,HSE_VALUE,PLLCLKFreq_Value,PLLI2SPCLKFreq_Value,PLLI2SQCLKFreq_Value,PLLI2SRCLKFreq_Value,PLLM,PLLN,PLLQCLKFreq_Value,PLLQoutputFreq_Value,PLLRCLKFreq_Value,PLLRoutputFreq_Value,PLLSourceVirtual,RNGFreq_Value,SDIOFreq_Value,SYSCLKSource,USBFreq_Value,VCOI2SInputFreq_Value,VCOI2SOutputFreq_Value,VCOInputFreq_Value,VCOOutputFreq_Value
+RCC.IPParameters=CortexFreq_Value,DFSDMFreq_Value,FamilyName,HSE_VALUE,PLLCLKFreq_Value,PLLI2SPCLKFreq_Value,PLLI2SQCLKFreq_Value,PLLI2SRCLKFreq_Value,PLLM,PLLN,PLLQCLKFreq_Value,PLLQoutputFreq_Value,PLLRCLKFreq_Value,PLLRoutputFreq_Value,RNGFreq_Value,SDIOFreq_Value,USBFreq_Value,VCOI2SInputFreq_Value,VCOI2SOutputFreq_Value,VCOInputFreq_Value,VCOOutputFreq_Value
 RCC.PLLCLKFreq_Value=50000000
 RCC.PLLI2SPCLKFreq_Value=96000000
 RCC.PLLI2SQCLKFreq_Value=96000000
@@ -233,10 +226,8 @@ RCC.PLLQCLKFreq_Value=50000000
 RCC.PLLQoutputFreq_Value=50000000
 RCC.PLLRCLKFreq_Value=50000000
 RCC.PLLRoutputFreq_Value=50000000
-RCC.PLLSourceVirtual=RCC_PLLSOURCE_HSE
 RCC.RNGFreq_Value=50000000
 RCC.SDIOFreq_Value=50000000
-RCC.SYSCLKSource=RCC_SYSCLKSOURCE_HSE
 RCC.USBFreq_Value=50000000
 RCC.VCOI2SInputFreq_Value=1000000
 RCC.VCOI2SOutputFreq_Value=192000000