stm32/ADC: Fix prescaler calculation to include max frequency.

embassy-stm32/CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - feat: stm32/usart: add `eager_reads` option to control if buffered readers return as soon as possible or after more data is available ([#4668](https://github.com/embassy-rs/embassy/pull/4668))
 - feat: stm32/usart: add `de_assertion_time` and `de_deassertion_time` config options
 - change: stm32/uart: BufferedUartRx now returns all available bytes from the internal buffer
+- fix: stm32/adc: Calculate the ADC prescaler in a way that it allows for the max frequency to be reached
 
 ## 0.4.0 - 2025-08-26
 

embassy-stm32/src/adc/adc4.rs

@@ -128,7 +128,7 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        let raw_prescaler = rcc::raw_prescaler(frequency.0, MAX_ADC_CLK_FREQ.0);
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,

embassy-stm32/src/adc/c0.rs

@@ -66,7 +66,7 @@ pub enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        let raw_prescaler = rcc::raw_prescaler(frequency.0, MAX_ADC_CLK_FREQ.0);
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,

embassy-stm32/src/adc/g4.rs

@@ -72,7 +72,7 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        let raw_prescaler = rcc::raw_prescaler(frequency.0, MAX_ADC_CLK_FREQ.0);
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,

embassy-stm32/src/adc/v2.rs

@@ -71,7 +71,7 @@ impl Prescaler {
         // Datasheet for both F4 and F7 specifies min frequency 0.6 MHz, typ freq. 30 MHz and max 36 MHz.
         #[cfg(not(stm32f2))]
         const MAX_FREQUENCY: Hertz = Hertz(36_000_000);
-        let raw_div = freq.0 / MAX_FREQUENCY.0;
+        let raw_div = rcc::raw_prescaler(freq.0, MAX_FREQUENCY.0);
         match raw_div {
             0..=1 => Self::Div2,
             2..=3 => Self::Div4,

embassy-stm32/src/adc/v4.rs

@@ -93,7 +93,7 @@ enum Prescaler {
 
 impl Prescaler {
     fn from_ker_ck(frequency: Hertz) -> Self {
-        let raw_prescaler = frequency.0 / MAX_ADC_CLK_FREQ.0;
+        let raw_prescaler = rcc::raw_prescaler(frequency.0, MAX_ADC_CLK_FREQ.0);
         match raw_prescaler {
             0 => Self::NotDivided,
             1 => Self::DividedBy2,

embassy-stm32/src/rcc/mod.rs

@@ -409,3 +409,33 @@ pub(crate) fn init_rcc(_cs: CriticalSection, config: Config) {
         }
     }
 }
+
+/// Calculate intermediate prescaler number used to calculate peripheral prescalers
+///
+/// This function is intended to calculate a number indicating a minimum division
+/// necessary to result in a frequency lower than the provided `freq_max`.
+///
+/// The returned value indicates the `val + 1` divider is necessary to result in
+/// the output frequency that is below the maximum provided.
+///
+/// For example:
+/// 0 = divider of 1 => no division necessary as the input frequency is below max
+/// 1 = divider of 2 => division by 2 necessary
+/// ...
+///
+/// The provided max frequency is inclusive. So if `freq_in == freq_max` the result
+/// will be 0, indicating that no division is necessary. To accomplish that we subtract
+/// 1 from the input frequency so that the integer rounding plays in our favor.
+///
+/// For example:
+/// Let the input frequency be 110 and the max frequency be 55.
+/// If we naiively do `110/55 = 2` the renult will indicate that we need a divider by 3
+/// which in reality will be rounded up to 4 as usually a 3 division is not available.
+/// In either case the resulting frequency will be either 36 or 27 which is lower than
+/// what we would want. The result should be 1.
+/// If we do the following instead `109/55 = 1` indicating that we need a divide by 2
+/// which will result in the correct 55.
+#[allow(unused)]
+pub(crate) fn raw_prescaler(freq_in: u32, freq_max: u32) -> u32 {
+    freq_in.saturating_sub(1) / freq_max
+}