Configuring LPTIM to generate autoreload interrupts on STM32L4

[RrW1]

Hi,

I am trying to learn how to use the LPTIM1 of the STM32L433 Nucleo-64 board (with the CPU: STM32L433RCTxP). I want to experiment on generating an interrupt every second with the LPTIM (I usually use a TIMx peripheral for this, but was interested because the LPTIM1 could also run in STOP modes).

I sort of have a working code where the LPTIM1 generates an interrupt periodically, although not at every second (or at a rate of 1Hz):

/* Note: SYSCLK, HCLK, PCLK1, PCLK2 are all 16MHz running on HSI */
static void MX_LPTIM1_Init(void)
{
  hlptim1.Instance = LPTIM1;
  hlptim1.Init.Clock.Source = LPTIM_CLOCKSOURCE_APBCLOCK_LPOSC;
  hlptim1.Init.Clock.Prescaler = LPTIM_PRESCALER_DIV128;
  hlptim1.Init.Trigger.Source = LPTIM_TRIGSOURCE_SOFTWARE;
  hlptim1.Init.OutputPolarity = LPTIM_OUTPUTPOLARITY_HIGH;
  hlptim1.Init.UpdateMode = LPTIM_UPDATE_IMMEDIATE;
  hlptim1.Init.CounterSource = LPTIM_COUNTERSOURCE_INTERNAL;
  hlptim1.Init.Input1Source = LPTIM_INPUT1SOURCE_GPIO;
  hlptim1.Init.Input2Source = LPTIM_INPUT2SOURCE_GPIO;
  if (HAL_LPTIM_Init(&hlptim1) != HAL_OK)
  {
    Error_Handler();
  }
 
  HAL_NVIC_SetPriority(LPTIM1_IRQn, 1, 0);
  HAL_NVIC_EnableIRQ(LPTIM1_IRQn);
 
  /* 65535 is the value loaded into LPTIM1's ARR register */
  HAL_LPTIM_Counter_Start_IT(&hlptim1, 65535);
}
 
/***  Code portion below in the interrupt C file ***/
 
static uint32_t counter = 0;
 
/**
  * @brief  Autoreload match callback in non blocking mode 
  * @param  hlptim : LPTIM handle
  * @retval None
  */
void HAL_LPTIM_AutoReloadMatchCallback(LPTIM_HandleTypeDef *hlptim)
{
    /* Blink onboard LED */
    if(counter >= 2)
    {
        /* TODO: Do some task here */
        Func1();
 
        counter = 0;
    }
    
    counter++;
}

With the inclusion of the if(counter>=2) block, Func1() executes periodically at a slightly higher rate than 1Hz. I measured the time duration between each function call with my phone's stopwatch and I would say that the duration between each function call is around 0.9 seconds. (The way I tested it was to blink the LED at interrupt execution and measure the blinking LEDs with a stopwatch. This procedure was pretty accurate when I was testing a 1 second interrupt execution with TIM2).

I am wondering what the equation for the interrupt execution frequency rates (Hz) looks like for the LPTIM peripherals, and how to set up the LPTIM peripherals to produce an interrupt periodically properly. Is there an example parameter where I could generate an interrupt every second?

I also noticed that from this datasheet, TIM6 does not use that much more current than LPTIM1 does in Low Power Run mode. Aside from the LPTIMs being able to run in STOP mode and not requiring HSI as the clock source, what is the difference between TIM and LPTIM? If my application only requires the STM32 microcontroller to operate in low power run mode, would using the TIM6 to generate periodical interrupts be better than using LPTIM1?

Any advice/help would be greatly appreciated. Thank you!

[TDK · Accepted solution]

The update rate is (timer clock) / (prescaler) / (ARR + 1). Which in this case comes out to about 1.9Hz. Doesn't look like you can slow it down more without lowering the timer clock.

[RrW1]

Thanks for the answer! In the code above, the timer clock source used is the APB1 clock source as LPTIM1 is connected to APB1. If I were to use LSI, does that mean I have to disable internal HSI, configure the LSI, and then activate the LSI clock?

I might have had an error in measurements with the timing. If the LPTIM rate is 1.9Hz, and the function triggers every other interrupts (at each second interrupt), that would mean that between each function, the duration would be around 1.05 seconds?

Is there also any difference in accuracy between TIM6 and LPTIM1 in low power run mode?

Thanks!

[TDK]

> If I were to use LSI, does that mean I have to disable internal HSI, configure the LSI, and then activate the LSI clock?

LSI can be active at the same time as HSI. You can only use one for the system clock though.

You can use LSI for LPTIM1 while still using HSI for the system clock. They won't be synced, but maybe that's not a problem.

> that would mean that between each function, the duration would be around 1.05 seconds?

That's what the math says. Typically better to use a scope or logic analyzer or something if you're concerned with accuracy. You could measure 10 (or 100) periods instead of just 1 with a stopwatch to significantly improve your accuracy.

> Is there also any difference in accuracy between TIM6 and LPTIM1 in low power run mode?

Doubtful. Clock sources are either on or off, they don't really have a "high accuracy" mode.

[RrW1]

Thanks for the response. I will try to play with the clock sources to lower down the frequency of the LPTIM1 interrupts. Thanks for the suggestion on using a Logic Analyzer to monitor the timing, I'll try that approach.