diff --git a/ports/stm32/sdram.c b/ports/stm32/sdram.c
index e2b29c56e2..b3c5bbeeed 100644
--- a/ports/stm32/sdram.c
+++ b/ports/stm32/sdram.c
@@ -39,6 +39,13 @@
 #define SDRAM_START_ADDRESS 0xD0000000
 #endif
 
+// Provides the MPU_REGION_SIZE_X value when passed the size of region in bytes
+// "m" must be a power of 2 between 32 and 4G (2**5 and 2**32) and this formula
+// computes the log2 of "m", minus 1
+#define MPU_REGION_SIZE(m) (((m) - 1) / (((m) - 1) % 255 + 1) / 255 % 255 * 8 + 7 - 86 / (((m) - 1) % 255 + 12) - 1)
+
+#define SDRAM_MPU_REGION_SIZE (MPU_REGION_SIZE(MICROPY_HW_SDRAM_SIZE))
+
 #ifdef FMC_SDRAM_BANK
 
 static void sdram_init_seq(SDRAM_HandleTypeDef
@@ -244,17 +251,33 @@ static void sdram_init_seq(SDRAM_HandleTypeDef
     /* Disable the MPU */
     HAL_MPU_Disable();
 
-    /* Configure the MPU attributes as Write-Through for External SDRAM */
+    /* Configure the MPU attributes for External SDRAM
+       Initially disable all access for the entire SDRAM memory space,
+       then enable access/caching for the size used
+    */
     MPU_InitStruct.Enable = MPU_REGION_ENABLE;
+    MPU_InitStruct.Number = MPU_REGION_NUMBER4;
     MPU_InitStruct.BaseAddress = SDRAM_START_ADDRESS;
-    MPU_InitStruct.Size = MPU_REGION_SIZE_256MB;
-    MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
+    MPU_InitStruct.Size = MPU_REGION_SIZE_512MB;
+    MPU_InitStruct.AccessPermission = MPU_REGION_NO_ACCESS;
     MPU_InitStruct.IsBufferable = MPU_ACCESS_NOT_BUFFERABLE;
-    MPU_InitStruct.IsCacheable = MPU_ACCESS_CACHEABLE;
+    MPU_InitStruct.IsCacheable = MPU_ACCESS_NOT_CACHEABLE;
     MPU_InitStruct.IsShareable = MPU_ACCESS_NOT_SHAREABLE;
-    MPU_InitStruct.Number = MPU_REGION_NUMBER0;
     MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL0;
     MPU_InitStruct.SubRegionDisable = 0x00;
+    MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_DISABLE;
+    HAL_MPU_ConfigRegion(&MPU_InitStruct);
+
+    MPU_InitStruct.Enable = MPU_REGION_ENABLE;
+    MPU_InitStruct.Number = MPU_REGION_NUMBER5;
+    MPU_InitStruct.BaseAddress = SDRAM_START_ADDRESS;
+    MPU_InitStruct.Size = SDRAM_MPU_REGION_SIZE;
+    MPU_InitStruct.AccessPermission = MPU_REGION_FULL_ACCESS;
+    MPU_InitStruct.IsBufferable = MPU_ACCESS_BUFFERABLE;
+    MPU_InitStruct.IsCacheable = MPU_ACCESS_CACHEABLE;
+    MPU_InitStruct.IsShareable = MPU_ACCESS_NOT_SHAREABLE;
+    MPU_InitStruct.TypeExtField = MPU_TEX_LEVEL1;
+    MPU_InitStruct.SubRegionDisable = 0x00;
     MPU_InitStruct.DisableExec = MPU_INSTRUCTION_ACCESS_ENABLE;
     HAL_MPU_ConfigRegion(&MPU_InitStruct);