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Problem programming TIM2 on STM32F303

Question asked by grimes.robert on Sep 30, 2016
Latest reply on Sep 30, 2016 by Clive One
I am trying to set up TIM2 as a free-running microsecond counter; no interrupts or anything fancy.  Basically, I want to be able to read the 32-bit CNT register and get microseconds; this is very handy for fine-grained delays and timing for hardware related issues.  The SystemCoreClock value is set to 72,000,000, which is correct; my FreeRTOS kernel is running at 1,000 Hz based off that, and if I toggle an LED every 500 (FreeRTOS) ticks, it blinks correctly at 1 Hz (on 0.5 seconds, off 0.5 seconds).  All is fine.

However, when I program TIM2 with a prescaler of (72 - 1), which should cause it to count at 1 MHz, it counts at half that rate; I verified this by noting the difference in TIM2.CNT from one second to the next, and the difference is 500,000!

Here is my code:
TIM_TimeBaseInitTypeDef tinit;
uint16_t prescaler = (uint16_t) ((SystemCoreClock ) / 1000000) - 1;
tinit.TIM_Prescaler = prescaler;
TIM_TimeBaseInit(TIM2, &tinit);

I have verified that the system clock is being set correctly, multiplying my 12 MHz HSE by 6 to get to 72 MHz; besides, FreeRTOS is using it to correctly set up a 1000 Hz tick for its use.  What am I missing that results in half the frequency I desire?