RTC drifting - a lot!

[Wassini]

I have bought a STL8L Discovery kit and have installed the example for showing a clock: RTC using LSE - but it drift appx. 5 seconds per minute.

Seems like a simple 555 timer - or a cheap old analog clock - is more precise.

[Peter BENSCH]

it looks as if you are not using the crystal-stabilized LSE, but by accident the RC-based LSI. Its frequency can vary between 26...56kHz, which corresponds to an RTC "error" of -20.6...+70.9%, i.e. about -12...+42s. Where did you find the sample program?

Regards

/Peter

[Wassini]

From http://wwwst.com/stm8l -> STM8l15x_StdPeriph_Lib.zip\Project\STM8L15x_StdPeriph_Examples\RTC\

CLK_DeInit();
 
CLK_LSEConfig(CLK_LSE_ON); /* Enable LSE */
 
while (CLK_GetFlagStatus(CLK_FLAG_LSERDY) == RESET); /* Wait for LSE clock to be ready */
 
LSE_StabTime(); /* wait for 1 second for the LSE Stabilisation */
 
CLK_RTCClockConfig(CLK_RTCCLKSource_LSE, CLK_RTCCLKDiv_1);	// Low Speed External 
 
CLK_PeripheralClockConfig(CLK_Peripheral_RTC, ENABLE);			// Enable RTC
 
RTC_DeInit();
 
RTC_StructInit(&rtc_init);
errStatus = RTC_Init(&rtc_init);	
 
RTC_DateStructInit(&rtc_date);
errStatus = RTC_SetDate(RTC_Format_BIN, &rtc_date);

[Tesla DeLorean]

Make sure you set the prescalers appropriately

RTC_InitStr.RTC_HourFormat = RTC_HourFormat_24;

RTC_InitStr.RTC_AsynchPrediv = 127;

RTC_InitStr.RTC_SynchPrediv = 255;

RTC_Init(&RTC_InitStr);

Suggest you read-back/dump the registers you currently run with to confirm the mode/settings, and that they are in fact correct and appropriate.

[Tesla DeLorean]

void init_rtc(void)
{
	RTC_InitTypeDef RTC_InitStr;
	RTC_TimeTypeDef RTC_TimeStr;
	RTC_DateTypeDef RTC_DateStr;
 
#if 0
	/* LSI */
	CLK_RTCClockConfig(CLK_RTCCLKSource_LSI, CLK_RTCCLKDiv_1);
	CLK_PeripheralClockConfig(CLK_Peripheral_RTC, ENABLE);
 
	RTC_InitStr.RTC_HourFormat = RTC_HourFormat_24;
	RTC_InitStr.RTC_AsynchPrediv = 0x7C;
	RTC_InitStr.RTC_SynchPrediv = 0x012F;
	RTC_Init(&RTC_InitStr);
#else
	/* LSE */
	CLK_LSEConfig(CLK_LSE_ON);
	while (CLK_GetFlagStatus(CLK_FLAG_LSERDY) == RESET);
	CLK_RTCClockConfig(CLK_RTCCLKSource_LSE, CLK_RTCCLKDiv_1);
	CLK_PeripheralClockConfig(CLK_Peripheral_RTC, ENABLE);
 
	RTC_InitStr.RTC_HourFormat = RTC_HourFormat_24;
	RTC_InitStr.RTC_AsynchPrediv = 127;
	RTC_InitStr.RTC_SynchPrediv = 255;
	RTC_Init(&RTC_InitStr);
#endif
 
	RTC_TimeStructInit(&RTC_TimeStr);
	RTC_TimeStr.RTC_Hours = 00;
	RTC_TimeStr.RTC_Minutes = 00;
	RTC_TimeStr.RTC_Seconds = 00;
	RTC_SetTime(RTC_Format_BIN, &RTC_TimeStr);
 
	RTC_DateStructInit(&RTC_DateStr);
	RTC_DateStr.RTC_WeekDay = RTC_Weekday_Tuesday;
	RTC_DateStr.RTC_Date = 19;
	RTC_DateStr.RTC_Month = RTC_Month_October;
	RTC_DateStr.RTC_Year = 21;
	RTC_SetDate(RTC_Format_BIN, &RTC_DateStr);
}
 
void rtc_get_time(char *buf)
{
	RTC_TimeTypeDef RTC_TimeStr;
 
	while (RTC_WaitForSynchro() != SUCCESS);
 
	RTC_GetTime(RTC_Format_BIN, &RTC_TimeStr);
 
	sprintf(buf, "%02d:%02d:%02d",
		RTC_TimeStr.RTC_Hours & 0xff, RTC_TimeStr.RTC_Minutes & 0xff, RTC_TimeStr.RTC_Seconds & 0xff);
}
 
void rtc_get_date(char *buf)
{
	RTC_DateTypeDef RTC_DateStr;
 
	while (RTC_WaitForSynchro() != SUCCESS);
 
	RTC_GetDate(RTC_Format_BIN, &RTC_DateStr);
 
	sprintf(buf, "%04d-%02d-%02d",
		(RTC_DateStr.RTC_Year & 0xff) + 2000, RTC_DateStr.RTC_Month & 0xff, RTC_DateStr.RTC_Date & 0xff);
}

[Peter BENSCH]

It looks like inspired by the example RTC_Calendar.

There are at least two possibilities: you either changed the clock assignment later, or the RTC automatically switched to LSI due to an error (see datasheet, section 3.5).

Regards

/Peter

[Wassini]

Hi. Sorry - have been working on other projects - but I still haven't got this working.

Now I have copied the code from the "Discovery Board Demo", but I still can figure out how to change to LSE

I have tried to remove all function calls and defined types just to see if this was causing any problems. When debugging I get an error in line 22.

// Switch the clock to LSE 
void init_LSE() 
{
 
 // CLK_SYSCLKDivConfig(CLK_SYSCLKDiv_1);
	// CLK_LSEConfig(CLK_LSE_ON);
 // CLK_SYSCLKSourceConfig(CLK_SYSCLKSource_LSE);	// Switch in LSE clock 
 // CLK_SYSCLKSourceSwitchCmd(ENABLE);
 
	CLK->CKDIVR = (uint8_t)(0x00); // Set the System clock divider to 1
	
	CLK->ECKCR &= (uint8_t)~0x04; // Reset LSEON bit
	CLK->ECKCR &= (uint8_t)~0x20; // Reset LSEBYP bit
	CLK->ECKCR |= (uint8_t)0x04;	// Configure LSE 
	
	delay_ms(2000);	
 
 while (!((CLK->ECKCR)& 0x08));	// Wait flag LSE ready 
 
	CLK->SWR = (uint8_t)0x08;				// Switch in LSE clock 
 
	CLK->SWCR |= (uint8_t)0x02; 		// Set SWEN bit - CLK_CKDIVR
 while (((CLK->SWCR)& 0x01)==0x01);
 
}