Incorrect counter value measuring time between interrupts

CarrotMan — Fri, 07 Jan 2022 14:03:07 GMT

I am using evkmimxrt1064_gpt_timer example project. The settings for counter are as follows:

GPT_SetClockDivider(EXAMPLE_GPT, 6); //PER clock, so 75MHz/6 = 12.5MHz GPT_SetOutputCompareValue(EXAMPLE_GPT, kGPT_OutputCompare_Channel1, 125); //125 counts, so 100kHz signal

In the interrupt routine I toggle a GPIO pin and try to measure time between interrupts using the files in attachment. I got these files here on NXP forum.

In the interrupt routine I have the following code:

void EXAMPLE_GPT_IRQHandler(void) { result = us_count() - start_time; start_time = us_count(); /* Clear interrupt flag.*/ GPT_ClearStatusFlags(EXAMPLE_GPT, kGPT_OutputCompare1Flag); GPIO_PortToggle(EXAMPLE_LED_GPIO, 1u << EXAMPLE_LED_GPIO_PIN); gptIsrFlag = true; #if defined __CORTEX_M && (__CORTEX_M == 4U || __CORTEX_M == 7U) __DSB(); #endif }

The issue is that the result fluctuates from 5 to 30 microseconds when I put a breakpoint at GPIO_PortToggle. However, looking at the actual hardware signal, I can clearly see the port being toggled at 10 microseconds:

CarrotMan_0-1641562253394.png

My question is what causes the incorrect values to appear in result variable in software?

P.S. I realized that checking this with edge trigger is not exhaustive, so I checked it with pulse trigger mode, and it always was within the range of 9.6 to 10.5 microseconds.

Best regards,

CarrotMan

Re: Incorrect counter value measuring time between interrupts

Omar_Anguiano — Fri, 14 Jan 2022 02:20:52 GMT

Hello
Hope you are well.
This issue might be cause by the code inside the IRQ Handler. I suggest you leave the code that clears/sets flags.
To calculate the time you can use a callback function so the result can be processed outside the handler.

If you have more questions do not hesitate to ask me.
Best regards,
Omar

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Incorrect counter value measuring time between interrupts

Re: Incorrect counter value measuring time between interrupts