diff --git a/Downstream/Inc/interrupts.h b/Downstream/Inc/interrupts.h
index 1dd3f3c..da3aa07 100644
--- a/Downstream/Inc/interrupts.h
+++ b/Downstream/Inc/interrupts.h
@@ -58,6 +58,10 @@ void SysTick_Handler(void);
 void DMA2_Stream2_IRQHandler(void);
 void DMA2_Stream3_IRQHandler(void);
 void OTG_FS_IRQHandler(void);
+void BusFault_Handler(void);
+
+void EnableOneBusFault(void);
+
 
 #ifdef __cplusplus
 }
diff --git a/Downstream/Src/interrupts.c b/Downstream/Src/interrupts.c
index 1e75965..720cee4 100644
--- a/Downstream/Src/interrupts.c
+++ b/Downstream/Src/interrupts.c
@@ -46,6 +46,9 @@ extern HCD_HandleTypeDef hhcd_USB_OTG_FS;
 extern DMA_HandleTypeDef hdma_spi1_rx;
 extern DMA_HandleTypeDef hdma_spi1_tx;
 
+uint8_t BusFaultAllowed = 0;
+
+
 /******************************************************************************/
 /*            Cortex-M4 Processor Interruption and Exception Handlers         */ 
 /******************************************************************************/
@@ -90,4 +93,23 @@ void OTG_FS_IRQHandler(void)
 }
 
 
+//This weird stuff is required when disabling flash writes.
+//The deliberate flash lockout will cause a bus fault that we need to process.
+void EnableOneBusFault(void)
+{
+    SCB->SHCSR = SCB_SHCSR_BUSFAULTENA_Msk;
+    BusFaultAllowed = 1;
+}
+
+void BusFault_Handler(void)
+{
+    if (BusFaultAllowed)
+    {
+        BusFaultAllowed = 0;
+        return;
+    }
+    while(1);
+}
+
+
 /************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/
diff --git a/Downstream/Src/main.c b/Downstream/Src/main.c
index 3509f53..a2e1ec2 100644
--- a/Downstream/Src/main.c
+++ b/Downstream/Src/main.c
@@ -40,21 +40,22 @@
 #include "downstream_statemachine.h"
 #include "downstream_spi.h"
 #include "led.h"
+#include "interrupts.h"
 
 
 /* Private function prototypes -----------------------------------------------*/
 void SystemClock_Config(void);
 static void GPIO_Init(void);
+void DisableFlashWrites(void);
 void CheckFirmwareMatchesHardware(void);
 
 
-
 int main(void)
 {
     //First things first!
+    DisableFlashWrites();
     CheckFirmwareMatchesHardware();
 
-
     /* Configure the system clock */
     SystemClock_Config();
 
@@ -76,6 +77,23 @@ int main(void)
 }
 
 
+void DisableFlashWrites(void)
+{
+    //Disable flash writes until the next reset
+    //This will cause a bus fault interrupt, so allow one now.
+    EnableOneBusFault();
+    FLASH->KEYR = 999;
+
+    //Confirm that flash cannot be unlocked
+    //This unlock attempt will also cause a bus fault.
+    EnableOneBusFault();
+    if ((FLASH->CR & FLASH_CR_LOCK) == 0) while(1);
+    FLASH->KEYR = FLASH_KEY1;
+    FLASH->KEYR = FLASH_KEY2;
+    if ((FLASH->CR & FLASH_CR_LOCK) == 0) while(1);
+}
+
+
 void CheckFirmwareMatchesHardware(void)
 {
     //Check we are running on the expected hardware:
diff --git a/Upstream/Inc/interrupts.h b/Upstream/Inc/interrupts.h
index 2ac7f10..4183c04 100755
--- a/Upstream/Inc/interrupts.h
+++ b/Upstream/Inc/interrupts.h
@@ -64,6 +64,9 @@ void SysTick_Handler(void);
 void DMA2_Stream2_IRQHandler(void);
 void DMA2_Stream3_IRQHandler(void);
 void EXTI3_IRQHandler(void);
+void BusFault_Handler(void);
+
+void EnableOneBusFault(void);
 
 
 #ifdef __cplusplus
diff --git a/Upstream/Src/interrupts.c b/Upstream/Src/interrupts.c
index 82ac923..3da4f86 100755
--- a/Upstream/Src/interrupts.c
+++ b/Upstream/Src/interrupts.c
@@ -51,6 +51,8 @@ extern PCD_HandleTypeDef    hpcd_USB_OTG_FS;
 extern DMA_HandleTypeDef    spiTxDmaHandle;
 extern DMA_HandleTypeDef    spiRxDmaHandle;
 
+uint8_t BusFaultAllowed = 0;
+
 
 /******************************************************************************/
 /*            Cortex-M4 Processor Interruption and Exception Handlers         */ 
@@ -95,5 +97,24 @@ void EXTI3_IRQHandler(void)
 /////////////////////////
 
 
+//This weird stuff is required when disabling flash writes.
+//The deliberate flash lockout will cause a bus fault that we need to process.
+void EnableOneBusFault(void)
+{
+    SCB->SHCSR = SCB_SHCSR_BUSFAULTENA_Msk;
+    BusFaultAllowed = 1;
+}
+
+void BusFault_Handler(void)
+{
+    if (BusFaultAllowed)
+    {
+        BusFaultAllowed = 0;
+        return;
+    }
+    while(1);
+}
+
+
 
 /************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/
diff --git a/Upstream/Src/main.c b/Upstream/Src/main.c
index e14b8e0..04c7c2f 100755
--- a/Upstream/Src/main.c
+++ b/Upstream/Src/main.c
@@ -41,6 +41,7 @@
 #include "led.h"
 #include "upstream_statemachine.h"
 #include "upstream_spi.h"
+#include "interrupts.h"
 
 
 /* Private variables ---------------------------------------------------------*/
@@ -49,7 +50,8 @@
 
 /* Private function prototypes -----------------------------------------------*/
 void SystemClock_Config(void);
-static void GPIO_Init(void);
+void GPIO_Init(void);
+void DisableFlashWrites(void);
 void CheckFirmwareMatchesHardware(void);
 
 
@@ -57,9 +59,9 @@ void CheckFirmwareMatchesHardware(void);
 int main(void)
 {
     //First things first!
+    DisableFlashWrites();
     CheckFirmwareMatchesHardware();
 
-
     /* Configure the system clock */
     SystemClock_Config();
 
@@ -81,6 +83,23 @@ int main(void)
 }
 
 
+void DisableFlashWrites(void)
+{
+    //Disable flash writes until the next reset
+    //This will cause a bus fault interrupt, so allow one now.
+    EnableOneBusFault();
+    FLASH->KEYR = 999;
+
+    //Confirm that flash cannot be unlocked
+    //This unlock attempt will also cause a bus fault.
+    EnableOneBusFault();
+    if ((FLASH->CR & FLASH_CR_LOCK) == 0) while(1);
+    FLASH->KEYR = FLASH_KEY1;
+    FLASH->KEYR = FLASH_KEY2;
+    if ((FLASH->CR & FLASH_CR_LOCK) == 0) while(1);
+}
+
+
 void CheckFirmwareMatchesHardware(void)
 {
     //Check we are running on the expected hardware: