Improve f247 PLL output frequency calculation precision

This will produce the correct result for all integer output frequencies
and a truncated result for all non-integer output frequencies instead of
accumulating an error.

This is done by introducing a time::Scale type which is just a rational
representing the numerator <-> denominator relationship of a scaling
factor register.

Author: EliteTK

embassy-stm32/CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add I2S support for STM32F1, STM32C0, STM32F0, STM32F3, STM32F7, STM32G0, STM32WL, STM32H5, STM32H7RS
 - fix: STM32: Prevent dropped DacChannel from disabling Dac peripheral if another DacChannel is still in scope ([#4577](https://github.com/embassy-rs/embassy/pull/4577))
 - feat: Added support for more OctoSPI configurations (e.g. APS6408 RAM) ([#4581](https://github.com/embassy-rs/embassy/pull/4581))
+- Improve the PLL output frequency calculation precision for the F2, F4, and F7
+  series.
 
 ## 0.4.0 - 2025-08-26
 

embassy-stm32/build.rs

@@ -1682,8 +1682,7 @@ fn main() {
 
         if is_rcc_name(e.name) {
             let enum_name = format_ident!("{}", e.name);
-            let mut muls = Vec::new();
-            let mut divs = Vec::new();
+            let mut nds = Vec::new();
             for v in e.variants {
                 let Ok(val) = parse_num(v.name) else {
                     panic!("could not parse mul/div. enum={} variant={}", e.name, v.name)
@@ -1692,29 +1691,29 @@ fn main() {
                 let variant = quote!(crate::pac::rcc::vals::#enum_name::#variant_name);
                 let num = val.num;
                 let denom = val.denom;
-                muls.push(quote!(#variant => self * #num / #denom,));
-                divs.push(quote!(#variant => self * #denom / #num,));
+                nds.push(quote!(#variant => Self { num: #num, denom: #denom },));
             }
 
             g.extend(quote! {
-                impl core::ops::Div<crate::pac::rcc::vals::#enum_name> for crate::time::Hertz {
-                    type Output = crate::time::Hertz;
-                    fn div(self, rhs: crate::pac::rcc::vals::#enum_name) -> Self::Output {
-                        match rhs {
-                            #(#divs)*
+                impl core::convert::From<crate::pac::rcc::vals::#enum_name> for crate::time::Scale {
+                    fn from(value: crate::pac::rcc::vals::#enum_name) -> Self {
+                        match value {
+                            #(#nds)*
                             #[allow(unreachable_patterns)]
                             _ => unreachable!(),
                         }
                     }
                 }
+                impl core::ops::Div<crate::pac::rcc::vals::#enum_name> for crate::time::Hertz {
+                    type Output = crate::time::Hertz;
+                    fn div(self, rhs: crate::pac::rcc::vals::#enum_name) -> Self::Output {
+                        self / crate::time::Scale::from(rhs)
+                    }
+                }
                 impl core::ops::Mul<crate::pac::rcc::vals::#enum_name> for crate::time::Hertz {
                     type Output = crate::time::Hertz;
                     fn mul(self, rhs: crate::pac::rcc::vals::#enum_name) -> Self::Output {
-                        match rhs {
-                            #(#muls)*
-                            #[allow(unreachable_patterns)]
-                            _ => unreachable!(),
-                        }
+                        self * crate::time::Scale::from(rhs)
                     }
                 }
             });

embassy-stm32/src/rcc/f247.rs

@@ -2,14 +2,15 @@ use stm32_metapac::flash::vals::Latency;
 
 #[cfg(any(stm32f413, stm32f423, stm32f412))]
 pub use crate::pac::rcc::vals::Plli2ssrc as Plli2sSource;
+use crate::pac::rcc::vals::Pllm;
 pub use crate::pac::rcc::vals::{
     Hpre as AHBPrescaler, Pllm as PllPreDiv, Plln as PllMul, Pllp as PllPDiv, Pllq as PllQDiv, Pllr as PllRDiv,
     Pllsrc as PllSource, Ppre as APBPrescaler, Sw as Sysclk,
 };
 #[cfg(any(stm32f4, stm32f7))]
 use crate::pac::PWR;
 use crate::pac::{FLASH, RCC};
-use crate::time::Hertz;
+use crate::time::{Hertz, Scale};
 
 // TODO: on some F4s, PLLM is shared between all PLLs. Enforce that.
 // TODO: on some F4s, add support for plli2s_src
@@ -393,7 +394,8 @@ fn init_pll(instance: PllInstance, config: Option<Pll>, input: &PllInput) -> Pll
 
     let in_freq = pll_src / pll.prediv;
     assert!(max::PLL_IN.contains(&in_freq));
-    let vco_freq = in_freq * pll.mul;
+    let pll_mulsrc = pll_src.0 as u64 * Scale::from(pll.mul);
+    let vco_freq = Hertz((pll_mulsrc / Scale::from(pll.prediv)) as u32);
     assert!(max::PLL_VCO.contains(&vco_freq));
 
     // stm32f2 plls are like swiss cheese
@@ -408,9 +410,13 @@ fn init_pll(instance: PllInstance, config: Option<Pll>, input: &PllInput) -> Pll
         }
     }
 
-    let p = pll.divp.map(|div| vco_freq / div);
-    let q = pll.divq.map(|div| vco_freq / div);
-    let r = pll.divr.map(|div| vco_freq / div);
+    fn div_pll<T: Into<Scale>>(pll_mulsrc: u64, prediv: Pllm, div: T) -> Hertz {
+        Hertz((pll_mulsrc / (Scale::from(prediv) * div.into())) as u32)
+    }
+
+    let p = pll.divp.map(|div| div_pll(pll_mulsrc, pll.prediv, div));
+    let q = pll.divq.map(|div| div_pll(pll_mulsrc, pll.prediv, div));
+    let r = pll.divr.map(|div| div_pll(pll_mulsrc, pll.prediv, div));
 
     macro_rules! write_fields {
         ($w:ident) => {

embassy-stm32/src/time.rs

@@ -136,3 +136,47 @@ impl From<MaybeHertz> for Option<Hertz> {
         }
     }
 }
+
+#[derive(Clone, Copy, Debug, Default)]
+/// Time scaling factor (multiplier/divider), may be an unreduced fraction
+pub(crate) struct Scale {
+    pub num: u32,
+    pub denom: u32,
+}
+
+impl Mul for Scale {
+    type Output = Scale;
+    fn mul(self, rhs: Self) -> Self::Output {
+        let num = self.num * rhs.num;
+        let denom = self.denom * rhs.denom;
+        Self { num, denom }
+    }
+}
+
+impl Mul<Scale> for Hertz {
+    type Output = Hertz;
+    fn mul(self, rhs: Scale) -> Self::Output {
+        self * rhs.num / rhs.denom
+    }
+}
+
+impl Div<Scale> for Hertz {
+    type Output = Hertz;
+    fn div(self, rhs: Scale) -> Self::Output {
+        self * rhs.denom / rhs.num
+    }
+}
+
+impl Mul<Scale> for u64 {
+    type Output = u64;
+    fn mul(self, rhs: Scale) -> Self::Output {
+        self * rhs.num as u64 / rhs.denom as u64
+    }
+}
+
+impl Div<Scale> for u64 {
+    type Output = u64;
+    fn div(self, rhs: Scale) -> Self::Output {
+        self * rhs.denom as u64 / rhs.num as u64
+    }
+}