STM32U031 LPTIM1 not starting with Single Start mode
I am using a STM32U031 part with register level configuration (no Cube libraries). The Timer should be straight forward to start in a single-shot mode but when I set the LPTIM_CR_SNGSTRT flag in the CR register, the CNT register never increments and the SNGSTART flag never shows up in the SFRs window when debugging.
If I run the timer in continuous mode, it works okay.
I know the reference manual talks about a delay after enabling the timer. I've tried adding a 100us delay as a test after enabling it and it didn't work. Besides the code below works with continuous mode and it also doesn't work when stepping through with the debugger.
// I also tried just clearing LPTIM_CR_ENABLE which didn't work. This was another approach that I think I saw in the reference manual. It doesn't work either.
RCC->APBRSTR1 |= RCC_APBRSTR1_LPTIM1RST;
RCC->APBRSTR1 &= ~RCC_APBRSTR1_LPTIM1RST;
// Only modify CFGR register when disabled
LPTIM1->CFGR &= ~LPTIM_CFGR_PRESC_Msk;
LPTIM1->CFGR |= (prescale_powers_of_2 << LPTIM_CFGR_PRESC_Pos);
// Tried with and without WAVE and TIMOUT as a test, no mix would work.
LPTIM1->CFGR|= (LPTIM_CFGR_WAVE | LPTIM_CFGR_TIMOUT);
/* enable the LPTMR and start single shot counting. Timer must be enabled before setiing registers*/
LPTIM1->CR |= LPTIM_CR_ENABLE;
__NOP();
(void)LPTIM1->CR;
LPTIM1->ARR = timer_counts;
LPTIM1->ICR = LPTIM_FLAG_DIEROK;
LPTIM1->DIER |= (LPTIM_DIER_ARRMIE | LPTIM_DIER_UEIE);
while((LPTIM1->ISR & LPTIM_FLAG_DIEROK) == 0 ); // Wait for flagt to be set indicating DIER has been updated.
LPTIM1->CR |= LPTIM_CR_SNGSTRT; // Start counting in single shot mode
//LPTIM1->CR |= LPTIM_CR_CNTSTRT);
- LPTIM1 is clocked from the LSI.
- Yes, the LSI is enabled and selected as the clock for LPTIM1.
- Yes, the peripheral clock for LPTIM1 is enabled.
- No, the system is not currently running in low power modes.
Am I missing something? These are brand new parts but I don't see anything in the errata.
FYI, I can't find a ST Library function to run a single-shot lptimer without setting it up in compare mode to control one of the channel's pins. HAL_LPTIM_SetOnce_Start_IT and HAL_LPTIM_OnePulse_Start_IT both assume I want to set the GPIO on a counter match.