STM32 RTC Digital Smooth Calibration

[jan]

Posted on June 20, 2017 at 16:36

Hi everybody,

I've got a custom made board with STM32L486VG on it. I'm trying to do a RTC calibration with external 1Hz clock based on

http://www.st.com/en/embedded-software/x-cube-rtc.html

. Everything is set up and working OK. Though, I'm getting slightly different measurements when I have MCO enabled or disabled.

With help of timer 3 & 2, the code counts a number of LSE cycles (768Hz) in 32 seconds and compares the value to the expected value.

void SystemClock_Config(void) {
RCC_OscInitTypeDef RCC_OscInitStruct;
RCC_ClkInitTypeDef RCC_ClkInitStruct;
RCC_PeriphCLKInitTypeDef PeriphClkInit;
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI | RCC_OSCILLATORTYPE_LSE;
RCC_OscInitStruct.LSEState = RCC_LSE_ON;
RCC_OscInitStruct.HSIState = RCC_HSI_ON;
RCC_OscInitStruct.HSICalibrationValue = 16;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
RCC_OscInitStruct.PLL.PLLM = 1;
RCC_OscInitStruct.PLL.PLLN = 10;
RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV7;
RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
Error_Handler();
RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_SYSCLK
| RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4) != HAL_OK)
Error_Handler();
PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_RTC;
PeriphClkInit.RTCClockSelection = RCC_RTCCLKSOURCE_LSE;
if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
Error_Handler();
HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_LSE, RCC_MCODIV_1);
//HAL_RCCEx_EnableLSCO(RCC_LSCOSOURCE_LSE);
HAL_RCCEx_DisableLSCO();
__HAL_RCC_PWR_CLK_ENABLE();
if (HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1) != HAL_OK)
Error_Handler();
HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq() / 1000);
HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
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static void calibrate(void) {
/* expected number of RTCCLK cycles during 32-seconds */
const uint32_t cycles_expected = 0x100000;
uint32_t smooth_calib_plus_pulses = RTC_SMOOTHCALIB_PLUSPULSES_RESET;
uint32_t smooth_calib_minus_pulses_value = 0x0;
if (Calibration1stITDone == RESET) {
Calibration1stITDone = SET;
printf('Calibrating...\n');
}
else {
printf('Cycles expected: %lu\n', cycles_expected);
printf('Cycles measured: %lu\n', TIM2->CCR1);
if (TIM2->CCR1 >= cycles_expected) {
smooth_calib_minus_pulses_value = TIM2->CCR1 - cycles_expected;
smooth_calib_plus_pulses = RTC_SMOOTHCALIB_PLUSPULSES_RESET;
printf('CALP - Add 512 pulses: NO\n');
}
else {
smooth_calib_minus_pulses_value = (RTC_SMOOTH_CALIB_MINUS_MASK + 1) - (cycles_expected - TIM2->CCR1);
smooth_calib_plus_pulses = RTC_SMOOTHCALIB_PLUSPULSES_SET;
printf('CALP - Add 512 pulses: YES\n');
}
if (smooth_calib_minus_pulses_value > RTC_SMOOTH_CALIB_MINUS_MASK) {
printf('Out of calibration range - please change crystal!\n\n');
}
else {
printf('CALM - Minus pulses: %lu\n', smooth_calib_minus_pulses_value);
printf('Done\n\n');
//HAL_RTCEx_SetSmoothCalib(&hrtc, RTC_SMOOTHCALIB_PERIOD_32SEC, smooth_calib_plus_pulses, smooth_calib_minus_pulses_value);
}
}
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Case 1: With MCO enabled the result is:

HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_LSE, RCC_MCODIV_1);�?

System Core Clock: 80000000

Calibrating... Cycles expected: 1048576 Cycles measured: 1048587 CALP - Add 512 pulses: NO CALM - Minus pulses: 11 Done

Case 2:

With MCO disabledthe result is:

HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_NOCLOCK, RCC_MCODIV_1);�?

System Core Clock: 80000000

Calibrating... Cycles expected: 1048576 Cycles measured: 1048590 CALP - Add 512 pulses: NO CALM - Minus pulses: 14 Done

Any ideas why the difference??

Kind regards,

Jan