STM32H743 RTC clock resets upon reset, but backup registers are retained?
I'm working on a custom board using an STM32H743IIT6. In general, the board is working, but I'm running into an issue with the RTC. The RTC is running accurately, but when I reset the board, the RTC resets. This happens even if I just terminate and restart the connected debugger, without disconnecting power. I've read through about 20 pages of RTC related posts here, and have checked all sorts of things:
- I commented out the Cube/HAL code that initializes the RTC to a specific value
- I checked the battery (1.6V with a 357 battery)
- I tried using a method described on this forum: After initializing the RTC, store a magic number to a backup register. If that magic number is found in the backup register on startup, don't initialize the RTC again. It didn't help. I took this code out, because the RTC itself has a flag that indicates whether it's been initialized or not, and the HAL code skips initialization if the RTC is already initialized, based on that flag.
- My init code looks like this:
(in main())
__HAL_RCC_BKPRAM_CLK_ENABLE();
HAL_PWR_EnableBkUpAccess();(in MX_RTC_Init())
static void MX_RTC_Init(void)
{
/* USER CODE BEGIN RTC_Init 0 */
/* USER CODE END RTC_Init 0 */
RTC_TimeTypeDef sTime = {0};
RTC_DateTypeDef sDate = {0};
/* USER CODE BEGIN RTC_Init 1 */
/* USER CODE END RTC_Init 1 */
/** Initialize RTC Only
*/
hrtc.Instance = RTC;
hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
hrtc.Init.AsynchPrediv = 127;
hrtc.Init.SynchPrediv = 255;
hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
hrtc.Init.OutPutRemap = RTC_OUTPUT_REMAP_NONE;
if (HAL_RTC_Init(&hrtc) != HAL_OK)
{
Error_Handler();
}
/* USER CODE BEGIN Check_RTC_BKUP */
// RJM: Don't set time and date here!
#if 0
/* USER CODE END Check_RTC_BKUP */
/** Initialize RTC and set the Time and Date
*/
sTime.Hours = 0;
sTime.Minutes = 0;
sTime.Seconds = 0;
sTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE;
sTime.StoreOperation = RTC_STOREOPERATION_RESET;
if (HAL_RTC_SetTime(&hrtc, &sTime, RTC_FORMAT_BIN) != HAL_OK)
{
Error_Handler();
}
sDate.WeekDay = RTC_WEEKDAY_MONDAY;
sDate.Month = RTC_MONTH_JANUARY;
sDate.Date = 1;
sDate.Year = 0;
if (HAL_RTC_SetDate(&hrtc, &sDate, RTC_FORMAT_BIN) != HAL_OK)
{
Error_Handler();
}
/* USER CODE BEGIN RTC_Init 2 */
#endif
// Wait for RTC to sync up with its shadow registers so that the first read actually works
(void)HAL_RTC_WaitForSynchro(&hrtc);
/* USER CODE END RTC_Init 2 */
}I've run this code in the debugger, putting a breakpoint on the first line of SystemInit(). The RTC is clearly running, but even at that first line, I can see that the time and date have been reset. I can read and write the RTC time all I want while the board is running, and it'll keep time correctly. The RTC registers are correct up until the board resets.
The other notable thing is that the magic number that I put in backup register 0 is still there, even though I removed the code that sets this value, and had the board unplugged for a good while. So, it seems that Vbat is good, but somehow the clock isn't being retained.
Does anyone have any suggestions on how to track this issue down?