Fix, enable PWR clock before accessing the PWR_CR

changelog

Author: apeng2012

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- RTC not enable PWR clock
 - USART2 remap
 - Fix > 2 byte i2c reads
 - Send stop after acknowledge errors on i2c

src/rcc.rs

@@ -327,6 +327,7 @@ impl BKP {
         // Enable the backup interface by setting PWREN and BKPEN
         let rcc = unsafe { &(*RCC::ptr()) };
         crate::pac::BKP::enable(rcc);
+        crate::pac::PWR::enable(rcc);
 
         // Enable access to the backup registers
         pwr.cr.modify(|_r, w| w.dbp().set_bit());