HAL_RCC_OscConfig takes too long (appx 170 μS), I need it to be <50 μS when waking from STOP

Question

Development for STM32L053R8 on NUCLEO-L053R8 board.

We have a system which "wakes" from sleep every 200 μS or so, does a small amount of work then goes back to sleep (Stop mode). Ideally I'd like to wake from STOP in under 50 μS. The HAL_RCC_OscConfig() function takes around 170 μS which renders this effort pointless.

From what I can see the majority of time is spent with the PLL Configuration, in particular the while loop ("Wait till PLL is ready") which follows the re-enablement of the PLL (about 98 μS).

/* Configure the main PLL clock source, multiplication and division factors. */
__HAL_RCC_PLL_CONFIG(RCC_OscInitStruct->PLL.PLLSource,
                     RCC_OscInitStruct->PLL.PLLMUL,
                     RCC_OscInitStruct->PLL.PLLDIV);
/* Enable the main PLL. */
__HAL_RCC_PLL_ENABLE();

/* Get timeout */
tickstart = HAL_GetTick();

/* Wait till PLL is ready */  
while(__HAL_RCC_GET_FLAG(RCC_FLAG_PLLRDY) == RESET)
{
  if((HAL_GetTick() - tickstart ) > RCC_PLL_TIMEOUT_VALUE)
  {
    return HAL_TIMEOUT;
  }      
}  

Are there any ways to wake from STOP mode and get back to full speed HSI in under 50 μS? What is the most efficient method to set the clocks when waking from STOP?

Currently I use the method prescribed in the PWR_STOP example which is as follows:

/* Enter Stop Mode */
HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);

/* Stop interrupt that woke us up */
int ret = HAL_LPTIM_TimeOut_Stop_IT(&LptimHandle);
SystemDisableWakeupCounter();

/* Configures system clock after wake-up from STOP: enable HSI, PLL and select
PLL as system clock source (HSI and PLL are disabled automatically in STOP mode) */
SystemClockConfig_STOP();

The call to SystemClockConfig_STOP() in turn calls SystemClock_Config() which configures the clocks and includes this long delay in HAL_RCC_OscConfig(&RCC_OscInitStruct).

Many thanks in advance for any assistance offered.

Answer 1

Well, the datasheet says PLL locking can take up to 160 μs, so the delay will be there as long as you are trying to use PLL.

Do you need PLL at all?

You can configure RCC to wake up with HSI already running at 16 MHz (see the RCC_CFGR_STOPWUCK bit). You get only half the speed, but it's up and running in 6 us.

Answer 2

I do not know what is your PLL source right now, but HSI16 is recommended. All from the reference manual.

The HSI16 clock signal is generated from an internal 16 MHz RC oscillator. It can be used directly as a system clock or as PLL input.

The HSI16 clock can be used after wake-up from the Stop low-power mode, this ensure a smaller wake-up time than a wake-up using MSI clock.

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If you can allow the slightly higher consumption, do not disable the internal regulator. It is a compromise: faster wake-up with higher consumption or slower while saving more energy, decide which is the more important.

When exiting Stop mode by issuing an interrupt or a wakeup event, the MSI or HSI16 RC oscillator is selected as system clock depending the bit STOPWUCK in the RCC_CFGR register.

When the voltage regulator operates in low-power mode, an additional startup delay is incurred when waking up from Stop mode. By keeping the internal regulator ON during Stop mode, the consumption is higher although the startup time is reduced.

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Now I suppose using the PLL is a must, but then I am afraid apart from the above mentioned you cannot do really much about the startup time. The PLL needs a certain time to lock onto the input reference clock frequency. But first the input frequency must become stable (that's why it is recommended to use the HSI16).

Instead of the blocking wait (while loop) you can use an interrupt.

An interrupt can be generated when the PLL is ready if enabled in the RCC_CIER register (see Section 7.3.5).

And while the PLL is locking you can do some tasks on 16 MHz, and the tasks requiring the full-speed can be started when the interrupt is received (finish clock configuration in the ISR).