diff --git a/embassy-stm32/src/flash/f4.rs b/embassy-stm32/src/flash/f4.rs
index 3d696223c..50ab446bd 100644
--- a/embassy-stm32/src/flash/f4.rs
+++ b/embassy-stm32/src/flash/f4.rs
@@ -71,7 +71,7 @@ mod alt_regions {
 
     impl<'d> Flash<'d> {
         pub fn into_alt_regions(self) -> AltFlashLayout<'d, Async> {
-            unsafe { crate::pac::FLASH.optcr().modify(|r| r.set_db1m(true)) };
+            super::set_alt_layout();
 
             // SAFETY: We never expose the cloned peripheral references, and their instance is not public.
             // Also, all async flash region operations are protected with a mutex.
@@ -88,7 +88,7 @@ mod alt_regions {
         }
 
         pub fn into_alt_blocking_regions(self) -> AltFlashLayout<'d, Blocking> {
-            unsafe { crate::pac::FLASH.optcr().modify(|r| r.set_db1m(true)) };
+            super::set_alt_layout();
 
             // SAFETY: We never expose the cloned peripheral references, and their instance is not public.
             // Also, all blocking flash region operations are protected with a cs.
@@ -171,12 +171,31 @@ static WAKER: AtomicWaker = AtomicWaker::new();
 
 #[cfg(any(stm32f427, stm32f429, stm32f437, stm32f439, stm32f469, stm32f479))]
 pub fn set_default_layout() {
-    unsafe { crate::pac::FLASH.optcr().modify(|r| r.set_db1m(false)) };
+    unsafe {
+        pac::FLASH.optkeyr().write(|w| w.set_optkey(0x08192A3B));
+        pac::FLASH.optkeyr().write(|w| w.set_optkey(0x4C5D6E7F));
+        pac::FLASH.optcr().modify(|r| {
+            r.set_db1m(false);
+            r.set_optlock(true)
+        });
+    };
 }
 
 #[cfg(not(any(stm32f427, stm32f429, stm32f437, stm32f439, stm32f469, stm32f479)))]
 pub const fn set_default_layout() {}
 
+#[cfg(any(stm32f427, stm32f429, stm32f437, stm32f439, stm32f469, stm32f479))]
+fn set_alt_layout() {
+    unsafe {
+        pac::FLASH.optkeyr().write(|w| w.set_optkey(0x08192A3B));
+        pac::FLASH.optkeyr().write(|w| w.set_optkey(0x4C5D6E7F));
+        pac::FLASH.optcr().modify(|r| {
+            r.set_db1m(true);
+            r.set_optlock(true)
+        });
+    };
+}
+
 #[cfg(any(stm32f427, stm32f429, stm32f437, stm32f439, stm32f469, stm32f479))]
 pub fn get_flash_regions() -> &'static [&'static FlashRegion] {
     if unsafe { pac::FLASH.optcr().read().db1m() } {
@@ -207,8 +226,8 @@ pub(crate) unsafe fn lock() {
 }
 
 pub(crate) unsafe fn unlock() {
-    pac::FLASH.keyr().write(|w| w.set_key(0x4567_0123));
-    pac::FLASH.keyr().write(|w| w.set_key(0xCDEF_89AB));
+    pac::FLASH.keyr().write(|w| w.set_key(0x45670123));
+    pac::FLASH.keyr().write(|w| w.set_key(0xCDEF89AB));
 }
 
 #[cfg(feature = "nightly")]