Merge pull request #5073 from tonarino/fix-config-init

[STM32U5] Source system clock from MSIS before (de)configuring PLLs

2 files changed, 11 insertions, 0 deletions

diff --git a/embassy-stm32/CHANGELOG.md b/embassy-stm32/CHANGELOG.md
index 38f22b1c3..d26f1acdd 100644
--- a/embassy-stm32/CHANGELOG.md
+++ b/embassy-stm32/CHANGELOG.md
@@ -93,6 +93,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - stm32: Add blocking_listen for blocking I2C driver
 - fix: stm32l47*/stm32l48* adc analog pin setup
 - fix: keep stm32/sai: make NODIV independent of MCKDIV
+- fix: Source system clock from MSIS before (de)configuring PLLs on STM32U5
 
 ## 0.4.0 - 2025-08-26
 
diff --git a/embassy-stm32/src/rcc/u5.rs b/embassy-stm32/src/rcc/u5.rs
index 7b0dcb63f..47cc29c6f 100644
--- a/embassy-stm32/src/rcc/u5.rs
+++ b/embassy-stm32/src/rcc/u5.rs
@@ -343,6 +343,16 @@ pub(crate) unsafe fn init(config: Config) {
 
     let hsi48 = config.hsi48.map(super::init_hsi48);
 
+    // There's a possibility that a bootloader that ran before us has configured the system clock
+    // source to be PLL1_R. In that case we'd get forever stuck on (de)configuring PLL1 as the chip
+    // prohibits disabling PLL1 when it's used as a source for system clock. Change the system
+    // clock source to MSIS which doesn't suffer from this conflict. The correct source per the
+    // provided config is then set further down.
+    // See https://github.com/embassy-rs/embassy/issues/5072
+    let default_system_clock_source = Config::default().sys;
+    RCC.cfgr1().modify(|w| w.set_sw(default_system_clock_source));
+    while RCC.cfgr1().read().sws() != default_system_clock_source {}
+
     let pll_input = PllInput { hse, hsi, msi: msis };
     let pll1 = init_pll(PllInstance::Pll1, config.pll1, &pll_input, config.voltage_range);
     let pll2 = init_pll(PllInstance::Pll2, config.pll2, &pll_input, config.voltage_range);