High current draw in STOP mode when RTC Alarm has been used

[Thomas Hoppe]

We have a very weird behavior in a battery powered IoT device. We should have a very low STOP mode current draw of 1.4 uA @ 3 VCC. The problem is, as soon as we use the RTC Alarm just once and although the alarm already fired during NORMAL operation, the current consumption is ~20 uA in STOP mode. The use of the wake up feature of the RTC does not cause this behavior, it's just alarms. Here is an example to reproduce this:

// Globals
volatile int alarmVal = 0;
void HAL_RTC_AlarmAEventCallback( RTC_HandleTypeDef *hrtc ) {
alarmVal = 1;
}

// Event Loop
while(1) {
RTC_AlarmTypeDef alarm;
RTC_TimeTypeDef ts;
RTC_DateTypeDef ds;

// Disable Alarm
HAL_RTC_DeactivateAlarm(&hrtc, RTC_ALARM_A); // Disable Interrupt
__HAL_RTC_ALARM_CLEAR_FLAG(&hrtc, RTC_FLAG_ALRAF); // Clear Flag (RTC ALARM A)
__HAL_RTC_ALARM_EXTI_CLEAR_FLAG(); // Clear Flag (EXTI)

// Get Time
do {
HAL_RTC_GetTime(&hrtc, &ts, FORMAT_BIN);
HAL_RTC_GetDate(&hrtc, &ds, FORMAT_BIN);
} while(ts.Seconds == 59); // cheat: Increment second without care of real clock

// Set Alarm
alarm.AlarmTime.SubSeconds = ts.SubSeconds; // 210
alarm.AlarmSubSecondMask = 8 << RTC_ALRMASSR_MASKSS_Pos; // 0x8000000
alarm.AlarmTime.Seconds = ts.Seconds + 1; // 27
alarm.AlarmTime.Minutes = ts.Minutes; // 0
alarm.AlarmTime.Hours = ts.Hours; // 0
alarm.AlarmDateWeekDay = ds.WeekDay; // 1
alarm.AlarmTime.TimeFormat = ts.TimeFormat; // 0
alarm.AlarmDateWeekDaySel = RTC_ALARMDATEWEEKDAYSEL_DATE; // 0
alarm.AlarmMask = RTC_ALARMMASK_NONE; // 0
alarm.Alarm = RTC_ALARM_A; // 0x100
alarm.AlarmTime.DayLightSaving = RTC_DAYLIGHTSAVING_NONE; // 0
alarm.AlarmTime.StoreOperation = RTC_STOREOPERATION_RESET; // 0
HAL_RTC_SetAlarm_IT(&hrtc, &alarm, RTC_FORMAT_BIN);

// Wait Alarm
for(alarmVal = 0; !alarmVal;);
HAL_RTC_DeactivateAlarm(&hrtc, RTC_ALARM_A);
__HAL_RTC_ALARM_CLEAR_FLAG(&hrtc, RTC_FLAG_ALRAF);
__HAL_RTC_ALARM_EXTI_CLEAR_FLAG();

// Stop Mode
HAL_RTC_MspDeInit(&hrtc);
HW_EnterStopMode();
HAL_RTC_MspInit(&hrtc);
}

We are using the LSE clock for the RTC

We have tried multiple ways to de-init the RTC before going to STOP mode but nothing helps.

How can this be explained and resolved?

BG

[KDJEM.1]

Hello @Thomas Hoppe ,

The typical current consumption in stop mode depends on the temperature and also on the RTC and Clocks.

• When the RTC disabled and all clocks OFF the typical current consumption, for VDD=3V, can be ranged between  0,43µA and 12.5 µA  when the temperature range [-40°C 125°C].
• When the RTC enabled and running on LSE, the typical current consumption increased and becomes range between 1µA and 14.1µA when the temperature range [-40°C 125°C], for VDD=3V.

So, the RTC is related to current consumption in stop mode. For more explanation, please take a look to the datasheet figure 18 and 19.

Thank you.  

[Thomas Hoppe · Accepted solution]

Hello @KDJEM.1 , this is not really the point of the problem and the demo I have provided. The point is that after the RTC triggered a wake-up, via Alarm or wake-up timer, the current draw was highly increased (20uA in our case). We have found the cause now which is that clearing of the Interrupt was missed:

--- Core/Src/main.c
+++ Core/Src/main.c
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {
if(GPIO_Pin == Button0_Pin)
__HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU);
}

void HW_EnterStopMode() {
HAL_ADC_DeInit(&hadc);
HAL_GPIO_WritePin(DC_Conv_Mode_GPIO_Port, DC_Conv_Mode_Pin, GPIO_PIN_RESET); // useful: Disable LEDs, I2C/SPI ICs, from 900 uA

HAL_PWREx_EnableUltraLowPower(); // useful: STOP Mode from 16.3 uA to 870 nA (when RTC ALARM never fired)
HAL_PWREx_EnableFastWakeUp(); // required: To wakeup via [PA0, GPIO_EXTI0, Button0, Button Press]

+ __HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU); // fixes the 20 uA
__HAL_RCC_PWR_CLK_ENABLE(); // Enable power control clock

hwSlept = 1;
HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
HW_ExitStopMode();
}

So with this addition in the demo code, it was solved. Can you confirm that?

[KDJEM.1]

Hi @Thomas Hoppe ,

Glad to know that the issue is solved and thank you for your sharing the solution this may help other users.

Entering low-power mode through WFI or WFE will be executed only is no interrupt and no event is pending.

Thank you for your contribution.

Kaouthar