1 files changed, 29 insertions, 17 deletions

diff --git a/embassy-stm32/src/usart/mod.rs b/embassy-stm32/src/usart/mod.rs
index e203336e1..255ddfd4b 100644
--- a/embassy-stm32/src/usart/mod.rs
+++ b/embassy-stm32/src/usart/mod.rs
@@ -809,45 +809,57 @@ fn configure(r: Regs, config: &Config, pclk_freq: Hertz, kind: Kind, enable_rx:
         Kind::Uart => (1, 0x10, 0x1_0000),
     };
 
+    fn calculate_brr(baud: u32, pclk: u32, presc: u32, mul: u32) -> u32 {
+        // The calculation to be done to get the BRR is `mul * pclk / presc / baud`
+        // To do this in 32-bit only we can't multiply `mul` and `pclk`
+        let clock = pclk / presc;
+
+        // The mul is applied as the last operation to prevent overflow
+        let brr = clock / baud * mul;
+
+        // The BRR calculation will be a bit off because of integer rounding.
+        // Because we multiplied our inaccuracy with mul, our rounding now needs to be in proportion to mul.
+        let rounding = ((clock % baud) * mul + (baud / 2)) / baud;
+
+        brr + rounding
+    }
+
     #[cfg(not(usart_v1))]
     let mut over8 = false;
-    let mut found = None;
+    let mut found_brr = None;
     for &(presc, _presc_val) in &DIVS {
-        let denom = (config.baudrate * presc as u32) as u64;
-        let div = (pclk_freq.0 as u64 * mul + (denom / 2)) / denom;
+        let brr = calculate_brr(config.baudrate, pclk_freq.0, presc as u32, mul);
         trace!(
             "USART: presc={}, div=0x{:08x} (mantissa = {}, fraction = {})",
             presc,
-            div,
-            div >> 4,
-            div & 0x0F
+            brr,
+            brr >> 4,
+            brr & 0x0F
         );
 
-        if div < brr_min {
+        if brr < brr_min {
             #[cfg(not(usart_v1))]
-            if div * 2 >= brr_min && kind == Kind::Uart && !cfg!(usart_v1) {
+            if brr * 2 >= brr_min && kind == Kind::Uart && !cfg!(usart_v1) {
                 over8 = true;
-                let div = div as u32;
-                r.brr().write_value(regs::Brr(((div << 1) & !0xF) | (div & 0x07)));
+                r.brr().write_value(regs::Brr(((brr << 1) & !0xF) | (brr & 0x07)));
                 #[cfg(usart_v4)]
                 r.presc().write(|w| w.set_prescaler(_presc_val));
-                found = Some(div);
+                found_brr = Some(brr);
                 break;
             }
             panic!("USART: baudrate too high");
         }
 
-        if div < brr_max {
-            let div = div as u32;
-            r.brr().write_value(regs::Brr(div));
+        if brr < brr_max {
+            r.brr().write_value(regs::Brr(brr));
             #[cfg(usart_v4)]
             r.presc().write(|w| w.set_prescaler(_presc_val));
-            found = Some(div);
+            found_brr = Some(brr);
             break;
         }
     }
 
-    let div = found.expect("USART: baudrate too low");
+    let brr = found_brr.expect("USART: baudrate too low");
 
     #[cfg(not(usart_v1))]
     let oversampling = if over8 { "8 bit" } else { "16 bit" };
@@ -857,7 +869,7 @@ fn configure(r: Regs, config: &Config, pclk_freq: Hertz, kind: Kind, enable_rx:
         "Using {} oversampling, desired baudrate: {}, actual baudrate: {}",
         oversampling,
         config.baudrate,
-        (pclk_freq.0 * mul as u32) / div
+        pclk_freq.0 / brr * mul
     );
 
     r.cr2().write(|w| {