diff --git a/ports/stm32f4/boards/feather_stm32f405_express/stm32f4xx_hal_conf.h b/ports/stm32f4/boards/feather_stm32f405_express/stm32f4xx_hal_conf.h
index 68a49b4ba8..8031403a91 100644
--- a/ports/stm32f4/boards/feather_stm32f405_express/stm32f4xx_hal_conf.h
+++ b/ports/stm32f4/boards/feather_stm32f405_express/stm32f4xx_hal_conf.h
@@ -55,7 +55,7 @@
 #define HAL_I2S_MODULE_ENABLED
 /* #define HAL_IWDG_MODULE_ENABLED   */
 /* #define HAL_LTDC_MODULE_ENABLED   */
-/* #define HAL_RNG_MODULE_ENABLED   */
+#define HAL_RNG_MODULE_ENABLED
 /* #define HAL_RTC_MODULE_ENABLED   */
 /* #define HAL_SAI_MODULE_ENABLED   */
 /* #define HAL_SD_MODULE_ENABLED   */
diff --git a/ports/stm32f4/boards/pyboard_v11/stm32f4xx_hal_conf.h b/ports/stm32f4/boards/pyboard_v11/stm32f4xx_hal_conf.h
index 7a1a9428f8..4dc18cb7f4 100644
--- a/ports/stm32f4/boards/pyboard_v11/stm32f4xx_hal_conf.h
+++ b/ports/stm32f4/boards/pyboard_v11/stm32f4xx_hal_conf.h
@@ -55,7 +55,7 @@
 #define HAL_I2S_MODULE_ENABLED
 /* #define HAL_IWDG_MODULE_ENABLED   */
 /* #define HAL_LTDC_MODULE_ENABLED   */
-/* #define HAL_RNG_MODULE_ENABLED   */
+#define HAL_RNG_MODULE_ENABLED
 /* #define HAL_RTC_MODULE_ENABLED   */
 /* #define HAL_SAI_MODULE_ENABLED   */
 /* #define HAL_SD_MODULE_ENABLED   */
diff --git a/ports/stm32f4/boards/stm32f412zg_discovery/stm32f4xx_hal_conf.h b/ports/stm32f4/boards/stm32f412zg_discovery/stm32f4xx_hal_conf.h
index b89240f982..f1b4013485 100644
--- a/ports/stm32f4/boards/stm32f412zg_discovery/stm32f4xx_hal_conf.h
+++ b/ports/stm32f4/boards/stm32f412zg_discovery/stm32f4xx_hal_conf.h
@@ -55,7 +55,7 @@
 #define HAL_I2S_MODULE_ENABLED
 /* #define HAL_IWDG_MODULE_ENABLED   */
 /* #define HAL_LTDC_MODULE_ENABLED   */
-/* #define HAL_RNG_MODULE_ENABLED   */
+#define HAL_RNG_MODULE_ENABLED
 /* #define HAL_RTC_MODULE_ENABLED   */
 /* #define HAL_SAI_MODULE_ENABLED   */
 #define HAL_SD_MODULE_ENABLED
diff --git a/ports/stm32f4/common-hal/os/__init__.c b/ports/stm32f4/common-hal/os/__init__.c
index aa80a02f5e..0b3153286a 100644
--- a/ports/stm32f4/common-hal/os/__init__.c
+++ b/ports/stm32f4/common-hal/os/__init__.c
@@ -30,6 +30,11 @@
 #include "py/objstr.h"
 #include "py/objtuple.h"
 
+#include "py/mperrno.h"
+#include "py/runtime.h"
+#include "stm32f4xx_hal.h"
+#include "stm32f4/periph.h"
+
 STATIC const qstr os_uname_info_fields[] = {
     MP_QSTR_sysname, MP_QSTR_nodename,
     MP_QSTR_release, MP_QSTR_version, MP_QSTR_machine
@@ -56,11 +61,35 @@ mp_obj_t common_hal_os_uname(void) {
     return (mp_obj_t)&os_uname_info_obj;
 }
 
-bool common_hal_os_urandom(uint8_t *buffer, uint32_t length) {
+#define RNG_TIMEOUT 5
 
-    for (uint32_t i = 0; i < length; i++) {
-        buffer[i] = 4; //read in my coffee dregs; the truest random number, chosen by Nrthalotep
-        //todo: spurn the gods, replace with actual code. 
+bool common_hal_os_urandom(uint8_t *buffer, uint32_t length) {
+    #if (HAS_TRNG)
+    //init the RNG
+    __HAL_RCC_RNG_CLK_ENABLE();
+    RNG_HandleTypeDef handle;
+    handle.Instance = RNG;
+    if (HAL_RNG_Init(&handle) != HAL_OK) mp_raise_ValueError(translate("RNG Init Error"));
+
+    //Assign bytes
+    for (uint i = 0; i < length; i++) {
+        uint32_t temp;
+        uint32_t start = HAL_GetTick();
+        //the HAL function has a timeout, but it isn't long enough, and isn't adjustable
+        while(!(__HAL_RNG_GET_FLAG(&handle,RNG_FLAG_DRDY)) && ((HAL_GetTick() - start) < RNG_TIMEOUT));
+        //
+        if (HAL_RNG_GenerateRandomNumber(&handle, &temp) != HAL_OK) {
+            mp_raise_ValueError(translate("Random number generation error"));
+        }
+        *buffer = (uint8_t)temp;
     }
+
+    //shut down the peripheral
+    if (HAL_RNG_DeInit(&handle) != HAL_OK) mp_raise_ValueError(translate("RNG DeInit Error"));
+    __HAL_RCC_RNG_CLK_DISABLE();
+
     return true;
+    #else
+    return false;
+    #endif
 }
\ No newline at end of file
diff --git a/ports/stm32f4/peripherals/stm32f4/periph.h b/ports/stm32f4/peripherals/stm32f4/periph.h
index 4b00ce1d92..0d1374e850 100644
--- a/ports/stm32f4/peripherals/stm32f4/periph.h
+++ b/ports/stm32f4/peripherals/stm32f4/periph.h
@@ -142,11 +142,13 @@ typedef struct {
 
 #ifdef STM32F411xE
 #define HAS_DAC 0
+#define HAS_TRNG 0
 #include "stm32f411xe/periph.h"
 #endif
 
 #ifdef STM32F412Zx
 #define HAS_DAC 0
+#define HAS_TRNG 1
 #include "stm32f412zx/periph.h"
 #endif
 
@@ -154,6 +156,7 @@ typedef struct {
 
 #ifdef STM32F405xx
 #define HAS_DAC 1
+#define HAS_TRNG 1
 #include "stm32f405xx/periph.h"
 #endif