RTC_ISR.INIT is not quite write protected by PWR_CR.DBP In 'F427 (and all RTCv2-containing STM32), some RTC registers are write protected: prior writing to them, it is necessary:1.enable backup domain access by setting PWR_CR.DBP2. unlock a RTC write protection by writing two key values into RTC_WPR Breakpoint 4, main () at rrr.c:37873
37873 int main(void) {
(gdb) p /x *RTC
$91 = {TR = 0x150002, DR = 0x258717, CR = 0x20, ISR = 0x17, PRER = 0x7f00ff, WUTR = 0xffff, CALIBR = 0x0,
(gdb) p /x *RTC
$92 = {TR = 0x150007, DR = 0x258717, CR = 0x20, ISR = 0x17, PRER = 0x7f00ff, WUTR = 0xffff, CALIBR = 0x0,
--- we see clock is running (RTC_CR.BYPSHAD = 1 so we don't need to worry about the readout locks)
(gdb) set RTC->ISR=RTC_ISR_INIT
(gdb) p /x *RTC
$93 = {TR = 0x150024, DR = 0x258717, CR = 0x20, ISR = 0x17, PRER = 0x7f00ff, WUTR = 0xffff, CALIBR = 0x0,
--- we see writing RTC_ISR_INIT had no effect, we haven't removed either of the protections
(gdb) set RCC->APB1ENR|=RCC_APB1ENR_PWREN
(gdb) set PWR->CR|=PWR_CR_DBP
(gdb) set RTC->
