9 files changed, 32 insertions, 22 deletions

diff --git a/embassy-stm32/CHANGELOG.md b/embassy-stm32/CHANGELOG.md
index a703a0c73..a9ab78e31 100644
--- a/embassy-stm32/CHANGELOG.md
+++ b/embassy-stm32/CHANGELOG.md
@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - fix: sdmmc: don't wait for DBCKEND flag on sdmmc_v2 devices as it never fires (Fixes #4723)
 - fix: usart: fix race condition in ringbuffered usart
 - feat: stm32/fdcan: add ability to control automatic recovery from bus off ([#4821](https://github.com/embassy-rs/embassy/pull/4821))
+- low-power: update rtc api to allow reconfig
 
 ## 0.4.0 - 2025-08-26
 
diff --git a/embassy-stm32/src/low_power.rs b/embassy-stm32/src/low_power.rs
index 1b66ca680..74cd11b93 100644
--- a/embassy-stm32/src/low_power.rs
+++ b/embassy-stm32/src/low_power.rs
@@ -43,8 +43,6 @@
 //!
 //!     // give the RTC to the executor...
 //!     let mut rtc = Rtc::new(p.RTC, RtcConfig::default());
-//!     static RTC: StaticCell<Rtc> = StaticCell::new();
-//!     let rtc = RTC.init(rtc);
 //!     embassy_stm32::low_power::stop_with_rtc(rtc);
 //!
 //!     // your application here...
@@ -98,10 +96,15 @@ pub(crate) unsafe fn on_wakeup_irq() {
 }
 
 /// Configure STOP mode with RTC.
-pub fn stop_with_rtc(rtc: &'static Rtc) {
+pub fn stop_with_rtc(rtc: Rtc) {
     unsafe { EXECUTOR.as_mut().unwrap() }.stop_with_rtc(rtc)
 }
 
+/// Reconfigure the RTC, if set.
+pub fn reconfigure_rtc(f: impl FnOnce(&mut Rtc)) {
+    unsafe { EXECUTOR.as_mut().unwrap() }.reconfigure_rtc(f);
+}
+
 /// Get whether the core is ready to enter the given stop mode.
 ///
 /// This will return false if some peripheral driver is in use that
@@ -181,14 +184,17 @@ impl Executor {
         trace!("low power: resume");
     }
 
-    pub(self) fn stop_with_rtc(&mut self, rtc: &'static Rtc) {
+    pub(self) fn stop_with_rtc(&mut self, rtc: Rtc) {
         self.time_driver.set_rtc(rtc);
-
-        rtc.enable_wakeup_line();
+        self.time_driver.reconfigure_rtc(|rtc| rtc.enable_wakeup_line());
 
         trace!("low power: stop with rtc configured");
     }
 
+    pub(self) fn reconfigure_rtc(&mut self, f: impl FnOnce(&mut Rtc)) {
+        self.time_driver.reconfigure_rtc(f);
+    }
+
     fn stop_mode(&self) -> Option<StopMode> {
         if unsafe { crate::rcc::REFCOUNT_STOP2 == 0 } && unsafe { crate::rcc::REFCOUNT_STOP1 == 0 } {
             Some(StopMode::Stop2)
diff --git a/embassy-stm32/src/rtc/low_power.rs b/embassy-stm32/src/rtc/low_power.rs
index 999f24714..9e0f03879 100644
--- a/embassy-stm32/src/rtc/low_power.rs
+++ b/embassy-stm32/src/rtc/low_power.rs
@@ -127,7 +127,7 @@ impl Rtc {
     /// start the wakeup alarm and with a duration that is as close to but less than
     /// the requested duration, and record the instant the wakeup alarm was started
     pub(crate) fn start_wakeup_alarm(
-        &self,
+        &mut self,
         requested_duration: embassy_time::Duration,
         cs: critical_section::CriticalSection,
     ) {
@@ -179,7 +179,10 @@ impl Rtc {
 
     /// stop the wakeup alarm and return the time elapsed since `start_wakeup_alarm`
     /// was called, otherwise none
-    pub(crate) fn stop_wakeup_alarm(&self, cs: critical_section::CriticalSection) -> Option<embassy_time::Duration> {
+    pub(crate) fn stop_wakeup_alarm(
+        &mut self,
+        cs: critical_section::CriticalSection,
+    ) -> Option<embassy_time::Duration> {
         let instant = self.instant().unwrap();
         if RTC::regs().cr().read().wute() {
             trace!("rtc: stop wakeup alarm at {}", instant);
diff --git a/embassy-stm32/src/rtc/v2.rs b/embassy-stm32/src/rtc/v2.rs
index 23f6ccb0c..8ac022536 100644
--- a/embassy-stm32/src/rtc/v2.rs
+++ b/embassy-stm32/src/rtc/v2.rs
@@ -93,7 +93,7 @@ impl super::Rtc {
         })
     }
 
-    pub(super) fn write<F, R>(&self, init_mode: bool, f: F) -> R
+    pub(super) fn write<F, R>(&mut self, init_mode: bool, f: F) -> R
     where
         F: FnOnce(crate::pac::rtc::Rtc) -> R,
     {
diff --git a/embassy-stm32/src/rtc/v3.rs b/embassy-stm32/src/rtc/v3.rs
index 01da5d70a..528dc78b4 100644
--- a/embassy-stm32/src/rtc/v3.rs
+++ b/embassy-stm32/src/rtc/v3.rs
@@ -95,7 +95,7 @@ impl super::Rtc {
         })
     }
 
-    pub(super) fn write<F, R>(&self, init_mode: bool, f: F) -> R
+    pub(super) fn write<F, R>(&mut self, init_mode: bool, f: F) -> R
     where
         F: FnOnce(crate::pac::rtc::Rtc) -> R,
     {
diff --git a/embassy-stm32/src/time_driver.rs b/embassy-stm32/src/time_driver.rs
index 74b10a183..7fd5751b3 100644
--- a/embassy-stm32/src/time_driver.rs
+++ b/embassy-stm32/src/time_driver.rs
@@ -213,7 +213,7 @@ pub(crate) struct RtcDriver {
     period: AtomicU32,
     alarm: Mutex<CriticalSectionRawMutex, AlarmState>,
     #[cfg(feature = "low-power")]
-    rtc: Mutex<CriticalSectionRawMutex, Cell<Option<&'static Rtc>>>,
+    rtc: Mutex<CriticalSectionRawMutex, RefCell<Option<Rtc>>>,
     queue: Mutex<CriticalSectionRawMutex, RefCell<Queue>>,
 }
 
@@ -221,7 +221,7 @@ embassy_time_driver::time_driver_impl!(static DRIVER: RtcDriver = RtcDriver {
     period: AtomicU32::new(0),
     alarm: Mutex::const_new(CriticalSectionRawMutex::new(), AlarmState::new()),
     #[cfg(feature = "low-power")]
-    rtc: Mutex::const_new(CriticalSectionRawMutex::new(), Cell::new(None)),
+    rtc: Mutex::const_new(CriticalSectionRawMutex::new(), RefCell::new(None)),
     queue: Mutex::new(RefCell::new(Queue::new()))
 });
 
@@ -379,7 +379,7 @@ impl RtcDriver {
     #[cfg(feature = "low-power")]
     /// Stop the wakeup alarm, if enabled, and add the appropriate offset
     fn stop_wakeup_alarm(&self, cs: CriticalSection) {
-        if let Some(offset) = self.rtc.borrow(cs).get().unwrap().stop_wakeup_alarm(cs) {
+        if let Some(offset) = self.rtc.borrow(cs).borrow_mut().as_mut().unwrap().stop_wakeup_alarm(cs) {
             self.add_time(offset, cs);
         }
     }
@@ -389,7 +389,7 @@ impl RtcDriver {
     */
     #[cfg(feature = "low-power")]
     /// Set the rtc but panic if it's already been set
-    pub(crate) fn set_rtc(&self, rtc: &'static Rtc) {
+    pub(crate) fn set_rtc(&self, mut rtc: Rtc) {
         critical_section::with(|cs| {
             rtc.stop_wakeup_alarm(cs);
 
@@ -398,6 +398,12 @@ impl RtcDriver {
     }
 
     #[cfg(feature = "low-power")]
+    /// Set the rtc but panic if it's already been set
+    pub(crate) fn reconfigure_rtc(&self, f: impl FnOnce(&mut Rtc)) {
+        critical_section::with(|cs| f(self.rtc.borrow(cs).borrow_mut().as_mut().unwrap()));
+    }
+
+    #[cfg(feature = "low-power")]
     /// The minimum pause time beyond which the executor will enter a low-power state.
     pub(crate) const MIN_STOP_PAUSE: embassy_time::Duration = embassy_time::Duration::from_millis(250);
 
@@ -418,7 +424,8 @@ impl RtcDriver {
             } else {
                 self.rtc
                     .borrow(cs)
-                    .get()
+                    .borrow_mut()
+                    .as_mut()
                     .unwrap()
                     .start_wakeup_alarm(time_until_next_alarm, cs);
 
diff --git a/examples/stm32h5/src/bin/stop.rs b/examples/stm32h5/src/bin/stop.rs
index 3c4f49f64..29a735ed2 100644
--- a/examples/stm32h5/src/bin/stop.rs
+++ b/examples/stm32h5/src/bin/stop.rs
@@ -39,8 +39,6 @@ async fn async_main(spawner: Spawner) {
 
     // give the RTC to the executor...
     let rtc = Rtc::new(p.RTC, RtcConfig::default());
-    static RTC: StaticCell<Rtc> = StaticCell::new();
-    let rtc = RTC.init(rtc);
     embassy_stm32::low_power::stop_with_rtc(rtc);
 
     spawner.spawn(unwrap!(blinky(p.PB4.into())));
diff --git a/examples/stm32l5/src/bin/stop.rs b/examples/stm32l5/src/bin/stop.rs
index c34053190..fd2aca372 100644
--- a/examples/stm32l5/src/bin/stop.rs
+++ b/examples/stm32l5/src/bin/stop.rs
@@ -30,8 +30,6 @@ async fn async_main(spawner: Spawner) {
 
     // give the RTC to the executor...
     let rtc = Rtc::new(p.RTC, RtcConfig::default());
-    static RTC: StaticCell<Rtc> = StaticCell::new();
-    let rtc = RTC.init(rtc);
     embassy_stm32::low_power::stop_with_rtc(rtc);
 
     spawner.spawn(unwrap!(blinky(p.PC7.into())));
diff --git a/tests/stm32/src/bin/stop.rs b/tests/stm32/src/bin/stop.rs
index 833ca05d0..cb3a5774c 100644
--- a/tests/stm32/src/bin/stop.rs
+++ b/tests/stm32/src/bin/stop.rs
@@ -70,9 +70,6 @@ async fn async_main(spawner: Spawner) {
 
     rtc.set_datetime(now.into()).expect("datetime not set");
 
-    static RTC: StaticCell<Rtc> = StaticCell::new();
-    let rtc = RTC.init(rtc);
-
     stop_with_rtc(rtc);
 
     spawner.spawn(task_1().unwrap());