Implement RTC peripheral for all stm32 families with rtc

embassy-stm32/src/rtc/v2/v2f4.rs

@@ -0,0 +1,31 @@
+use stm32_metapac::rcc::vals::Rtcsel;
+
+pub const BACKUP_REGISTER_COUNT: usize = 20;
+
+/// Unlock the backup domain
+pub(super) unsafe fn unlock_backup_domain(clock_config: u8) {
+    crate::pac::PWR.cr1().modify(|w| w.set_dbp(true));
+    while !crate::pac::PWR.cr1().read().dbp() {}
+
+    let reg = crate::pac::RCC.bdcr().read();
+
+    if !reg.rtcen() || reg.rtcsel().0 != clock_config {
+        crate::pac::RCC.bdcr().modify(|w| w.set_bdrst(true));
+
+        crate::pac::RCC.bdcr().modify(|w| {
+            // Reset
+            w.set_bdrst(false);
+
+            // Select RTC source
+            w.set_rtcsel(Rtcsel(clock_config));
+            w.set_rtcen(true);
+
+            w.set_lseon(reg.lseon());
+            w.set_lsebyp(reg.lsebyp());
+        });
+    }
+}
+
+pub(crate) unsafe fn enable_peripheral_clk() {
+    // Nothing to do
+}