Merge #767

767: Stm32 fixes r=lulf a=jr-oss

Using embassy on STM32 Discovery board showed problems with
- USART with parity
- Clock settings (CFGR) when using PLL and prescaler
- Flash ACR settings

This PR attempts to fix these


Co-authored-by: Ralf <jr-oss@gmx.net>

embassy-stm32/src/rcc/f3.rs

@@ -93,7 +93,10 @@ pub(crate) unsafe fn init(config: Config) {
     assert!(pclk2 <= 72_000_000);
 
     // Set latency based on HCLK frquency
-    FLASH.acr().write(|w| {
+    // RM0316: "The prefetch buffer must be kept on when using a prescaler
+    // different from 1 on the AHB clock.", "Half-cycle access cannot be
+    // used when there is a prescaler different from 1 on the AHB clock"
+    FLASH.acr().modify(|w| {
         w.set_latency(if hclk <= 24_000_000 {
             Latency::WS0
         } else if hclk <= 48_000_000 {
@@ -101,11 +104,16 @@ pub(crate) unsafe fn init(config: Config) {
         } else {
             Latency::WS2
         });
+        if hpre_div != 1 {
+            w.set_hlfcya(false);
+            w.set_prftbe(true);
+        }
     });
 
     // Enable HSE
+    // RM0316: "Bits 31:26 Reserved, must be kept at reset value."
     if config.hse.is_some() {
-        RCC.cr().write(|w| {
+        RCC.cr().modify(|w| {
             w.set_hsebyp(config.bypass_hse);
             // We turn on clock security to switch to HSI when HSE fails
             w.set_csson(true);
@@ -115,27 +123,30 @@ pub(crate) unsafe fn init(config: Config) {
     }
 
     // Enable PLL
+    // RM0316: "Reserved, must be kept at reset value."
     if let Some(ref pll_config) = pll_config {
-        RCC.cfgr().write(|w| {
+        RCC.cfgr().modify(|w| {
             w.set_pllmul(pll_config.pll_mul);
             w.set_pllsrc(pll_config.pll_src);
         });
         if let Some(pll_div) = pll_config.pll_div {
-            RCC.cfgr2().write(|w| w.set_prediv(pll_div));
+            RCC.cfgr2().modify(|w| w.set_prediv(pll_div));
         }
         RCC.cr().modify(|w| w.set_pllon(true));
         while !RCC.cr().read().pllrdy() {}
     }
 
+    // CFGR has been written before (PLL) don't overwrite these settings
     if config.pll48 {
         let usb_pre = get_usb_pre(&config, sysclk, pclk1, &pll_config);
-        RCC.cfgr().write(|w| {
+        RCC.cfgr().modify(|w| {
             w.set_usbpre(usb_pre);
         });
     }
 
     // Set prescalers
-    RCC.cfgr().write(|w| {
+    // CFGR has been written before (PLL, PLL48) don't overwrite these settings
+    RCC.cfgr().modify(|w| {
         w.set_ppre2(ppre2_bits);
         w.set_ppre1(ppre1_bits);
         w.set_hpre(hpre_bits);
@@ -146,7 +157,8 @@ pub(crate) unsafe fn init(config: Config) {
     //  1 to 16 AHB cycles after write"
     cortex_m::asm::delay(16);
 
-    RCC.cfgr().write(|w| {
+    // CFGR has been written before (PLL, PLL48, clock divider) don't overwrite these settings
+    RCC.cfgr().modify(|w| {
         w.set_sw(match (pll_config, config.hse) {
             (Some(_), _) => Sw::PLL,
             (None, Some(_)) => Sw::HSE,

embassy-stm32/src/usart/mod.rs

@@ -219,7 +219,11 @@ impl<'d, T: Instance, TxDma, RxDma> Uart<'d, T, TxDma, RxDma> {
                 w.set_ue(true);
                 w.set_te(true);
                 w.set_re(true);
-                w.set_m0(vals::M0::BIT8);
+                w.set_m0(if config.parity != Parity::ParityNone {
+                    vals::M0::BIT9
+                } else {
+                    vals::M0::BIT8
+                });
                 w.set_pce(config.parity != Parity::ParityNone);
                 w.set_ps(match config.parity {
                     Parity::ParityOdd => vals::Ps::ODD,