2 mS timer immideately fires interrupt on a first run

Posted on August 10, 2016 at 04:53

I am using Timer6 on STM32L051 as a base 2 S timer with overflow interrupt.

By some reason on a first run interrupt fires immediately, and I can't figure out the reason for this behavior. Here are my initialization and timer start functions. Initialization:

/**************************************************************** Timer6_init **
** Name : Timer6_init() 
** Returns : none
** Args : none
** Description : configure timer6 as a base timer
** Notes : auto-reload register is loaded with amount for 2S
** Timer6 is fed by 16 MHz clock
** timer is used as upcounter (DIR=0).
** Only overflow generates an update event (URS=1), 
** update event generates interrupt 
*******************************************************************************/
void Timer6_init(void)
{
/* enable TIM6 clock */
RCC->APB1ENR |= RCC_APB1ENR_TIM6EN; /* init timer 6 */
/* time base:
 TIM6_Prescaler = 15999 (count frequency is 1 KHz)
 TIM_ClockDivision = 0
 TIM_CounterMode = 0 (up counter */
/* update on overflow only */
/* time base is 1mS - 1 KHz */
TIM6->PSC = 15999;
TIM6->ARR = 2000; // in mS
TIM6->SR &= ~0x0001;
TIM6->CR1 |= 0x0004; // update source regular
/* enable overflow interrupt */ 
TIM6->DIER |= 0x0001;
} /* end Timer6_init() */

Timer start:

/***************************************************************** Start_tim6 **

** Name : Start_tim6

** Returns : none 

** Args : none

** Description : timer 6 - base milliseconds timer

** Notes : 

*******************************************************************************/

void Start_tim6(void)

{

TIM6->CNT = 0;

TIM6->ARR = 2000; // in mS - not needed, just in case 

TIM6->SR &= ~0x0001; /* clear update flag */

/* start timer */

TIM6->CR1 |= 0x0001;

if (first_run != 0)

{ 

TIM6->SR &= ~0x0001;

first_run = 0;

} 

__NOP();


} /* end Start_tim6() */

When timer runs for the first time, interrupt is fired immediately. I tried to set breakpoints in a timer start routine to see when the overflow flag is set. In the above Start_tim6 snippet it worked as follows: - Breakpoint at line 14 - SR is 0, - Breakpoint at line15 (after timer start) - SR is 1. I tried to clear flag on first run (lines 15-19). Flag is not cleared, it stays set. And, of coarse, interrupt is fired. This happens only on a first run. Every time after that interrupt occurs after 2 S. I wonder what could I overlook. Please help me to figure it out. Thank you, Gennady