stm32/timer: avoid max_compare_value >= u16::MAX

With STM32 32 bits timers, the max_compare_value (AKA, ARR register)
can currently be set greater than u16::MAX, which leads to the following
assert!(max < u16::MAX as u32) in max_duty_cycle() when setting up a 1 kHz
SimplePwm on 84 MHz MCU.

The issue is fixed by forcing a max_compare_value that fits into 16 bits
when setting the frequency for a PWM.

3 files changed, 18 insertions, 10 deletions

diff --git a/embassy-stm32/src/timer/complementary_pwm.rs b/embassy-stm32/src/timer/complementary_pwm.rs
index 46ccbf3df..02c01e900 100644
--- a/embassy-stm32/src/timer/complementary_pwm.rs
+++ b/embassy-stm32/src/timer/complementary_pwm.rs
@@ -116,7 +116,7 @@ impl<'d, T: AdvancedInstance4Channel> ComplementaryPwm<'d, T> {
         } else {
             1u8
         };
-        self.inner.set_frequency(freq * multiplier);
+        self.inner.set_frequency_internal(freq * multiplier, 16);
     }
 
     /// Get max duty value.
diff --git a/embassy-stm32/src/timer/low_level.rs b/embassy-stm32/src/timer/low_level.rs
index 3136ea4e9..7360d6aef 100644
--- a/embassy-stm32/src/timer/low_level.rs
+++ b/embassy-stm32/src/timer/low_level.rs
@@ -242,16 +242,28 @@ impl<'d, T: CoreInstance> Timer<'d, T> {
     /// In center-aligned mode (which not all timers support), the wrap-around frequency is effectively halved
     /// because it needs to count up and down.
     pub fn set_frequency(&self, frequency: Hertz) {
+        match T::BITS {
+            TimerBits::Bits16 => {
+                self.set_frequency_internal(frequency, 16);
+            }
+            #[cfg(not(stm32l0))]
+            TimerBits::Bits32 => {
+                self.set_frequency_internal(frequency, 32);
+            }
+        }
+    }
+
+    pub(crate) fn set_frequency_internal(&self, frequency: Hertz, max_divide_by_bits: u8) {
         let f = frequency.0;
         assert!(f > 0);
         let timer_f = T::frequency().0;
 
+        let pclk_ticks_per_timer_period = (timer_f / f) as u64;
+        let psc: u16 = unwrap!(((pclk_ticks_per_timer_period - 1) / (1 << max_divide_by_bits)).try_into());
+        let divide_by = pclk_ticks_per_timer_period / (u64::from(psc) + 1);
+
         match T::BITS {
             TimerBits::Bits16 => {
-                let pclk_ticks_per_timer_period = timer_f / f;
-                let psc: u16 = unwrap!(((pclk_ticks_per_timer_period - 1) / (1 << 16)).try_into());
-                let divide_by = pclk_ticks_per_timer_period / (u32::from(psc) + 1);
-
                 // the timer counts `0..=arr`, we want it to count `0..divide_by`
                 let arr = unwrap!(u16::try_from(divide_by - 1));
 
@@ -265,10 +277,6 @@ impl<'d, T: CoreInstance> Timer<'d, T> {
             }
             #[cfg(not(stm32l0))]
             TimerBits::Bits32 => {
-                let pclk_ticks_per_timer_period = (timer_f / f) as u64;
-                let psc: u16 = unwrap!(((pclk_ticks_per_timer_period - 1) / (1 << 32)).try_into());
-                let divide_by = pclk_ticks_per_timer_period / (u64::from(psc) + 1);
-
                 // the timer counts `0..=arr`, we want it to count `0..divide_by`
                 let arr: u32 = unwrap!(u32::try_from(divide_by - 1));
 
diff --git a/embassy-stm32/src/timer/simple_pwm.rs b/embassy-stm32/src/timer/simple_pwm.rs
index 9e4a09095..56fb1871e 100644
--- a/embassy-stm32/src/timer/simple_pwm.rs
+++ b/embassy-stm32/src/timer/simple_pwm.rs
@@ -278,7 +278,7 @@ impl<'d, T: GeneralInstance4Channel> SimplePwm<'d, T> {
         } else {
             1u8
         };
-        self.inner.set_frequency(freq * multiplier);
+        self.inner.set_frequency_internal(freq * multiplier, 16);
     }
 
     /// Get max duty value.