trying to understand systick and timer at STM32L1xx

[zoharr]

Posted on March 19, 2014 at 08:26

Hello,

I'm using STM32L152 discovery board.

I'm trying to define systick for 100mSec, and tim2 to 1 sec.

After measuring the pin togeling with scope, this is what I came up with:

I know SystemCoreClock is 32MHz.

For systick using 100mSec interrupt,

I tried using SysTick_Config(SystemCoreClock / 100) to get 100mSec,

but adjusting with scope I ended with

SysTick_Config(SystemCoreClock / 153) to get accurate 100mSec.

For tim2 at 1 sec interrupt, I tried using:

Prescaler = (TIM_CLK / TIM counter clock) - 1 = 32MHz / 32KHz - 1= 1000 - 1

Period = (TIM counter clock / timer output clock) - 1 = (32KHz / 1Hz) - 1 = 32000 -1

but adjusting with scope I came up with 

Prescaler = 1000 - 1

Period=2092 - 1

to get 1 sec interrupt.

Can you please clarify what am I doing wrong? what I don't understand?

Thanks

#mco

[stm322399]

Posted on March 19, 2014 at 10:28

Let me try a guess:

1) 32000000/153 cycles generates 10Hz signal ? you run 2,0915...MHz source clock

2) TIM1 generates 1Hz signal after being divided by 1000*2092, so you run 2,092MHz clock

3) STM32F152 has MSI clocked by default at 2,097MHz according the RM

That's too much for simple coincidence, isn't it ?

If I were you, I'll start to check my system initialization code.

good luck.

[os_kopernika]

Posted on March 22, 2014 at 18:00

+1

''I know SystemCoreClock is 32MHz.''

At least now you know you do not know.

Read about MCO.

[Tesla DeLorean]

Posted on March 22, 2014 at 18:18

I'd give the values returned by RCC_GetClocksFreq() more credibility than SystemCoreClock, but the MCO pin is the way to go to verify internal clocks.

/**
* @brief Returns the frequencies of the System, AHB and APB busses clocks.
* @note - The frequency returned by this function is not the real frequency
* in the chip. It is calculated based on the predefined constant and
* the source selected by RCC_SYSCLKConfig():
*
* - If SYSCLK source is MSI, function returns constant the MSI value
* as defined by the MSI range, refer to RCC_MSIRangeConfig()
*
* - If SYSCLK source is HSI, function returns constant HSI_VALUE(*)
*
* - If SYSCLK source is HSE, function returns constant HSE_VALUE(**)
*
* - If SYSCLK source is PLL, function returns constant HSE_VALUE(**)
* or HSI_VALUE(*) multiplied/divided by the PLL factors.
*
* (*) HSI_VALUE is a constant defined in stm32l1xx.h file (default value
* 16 MHz) but the real value may vary depending on the variations
* in voltage and temperature, refer to RCC_AdjustHSICalibrationValue().
*
* (**) HSE_VALUE is a constant defined in stm32l1xx.h file (default value
* 8 MHz), user has to ensure that HSE_VALUE is same as the real
* frequency of the crystal used. Otherwise, this function may
* return wrong result.
*
* - The result of this function could be not correct when using fractional
* value for HSE crystal.
*
* @param RCC_Clocks: pointer to a RCC_ClocksTypeDef structure which will hold
* the clocks frequencies.
* @retval None
*/
void RCC_GetClocksFreq(RCC_ClocksTypeDef* RCC_Clocks)
{
..

[AKhan.17]

time between 2 ticks = 100 ms so the reload value would be= 32000000*100ms-1 or 32000000*.01-1 which is 319999. can you please explain me why your are dividing 32Mhz by 100ms