Question about high resolution timer

[Stanley]

Hi,

I'd like to get a timer with 1ms interval or higher resolution, and found some questions about this:

1. If the app_timer is used, looks like we only can get minimum interval 5ms. With the less interval, it causes the system crashed and reboot. BTW, to use the 5ms interval, I updated the example macro MS_TO_TICK to below:

   #define MS_TO_TICK(MS) ((TICKS_PER_100MS * MS) / 100)

Is 5ms as the resolution for app_timer expected? If it's using 32K RTC as the clock source, I suppose the resolution should be much higher, right?

2. I turned to solution with the timer in TIMER mode as the example suggested, but observed some confusing issues:

For easier usage, I referred the code of app_timer to define below macros:

#define ROUNDED_DIV(A, B) (((A) + ((B) / 2)) / (B))
#define TIMER_PRESCALER                 6
#define TIMER_FREQUENCY                 (256 * 1024)   /* 16M / exp(2, PRESCALER) */
#define TIMER_TICKS_PER_MS              ROUNDED_DIV(TIMER_FREQUENCY, 1000)
#define TIMER_MS_TO_TICK(MS)            (TIMER_TICKS_PER_MS * MS)

and set CC as below:

NRF_TIMER2->CC[0] = TIMER_MS_TO_TICK(100);

But from the logs I added in timer handler, this setting gets 25ms interval instead of 100ms. What am I missing for this setting?

Another issue about timer is that whatever I set for BITMODE, 16 bit or 32 bit, the max CC value is always (64K - 1). If this is true, it will limit the max interval I'm trying to set. Could you please confirm this?

Thanks & Regards, Stanley

[Stanley]

With more tests, I tried to set different value for CC[0] directly, (Interrupt on CC[1] is disabled), and with below interrupt handler, looks like I got same timeout interval:

void TIMER2_IRQHandler(void)
{
        if ((NRF_TIMER2->EVENTS_COMPARE[0] != 0) && ((NRF_TIMER2->INTENSET & TIMER_INTENSET_COMPARE0_Msk) != 0)) {
                static int count = 0;
                count++;
                NRF_TIMER2->EVENTS_COMPARE[0] = 0;           //Clear compare register 0 event   
                if (count % 10 == 0) {
                        printf("%d\r\n", count);
                        nrf_gpio_pin_toggle(GPIO_TOGGLE_PIN);           //Set LED
                }
        }
}

Could anyone help me on this question? Any feedback will be really appreciated.

Thanks & Regards, Stanley

[Susheel Nuguru]

Hi Stanley,

Timer1 and Timer2 only support 16bit and these are the only timers accessable by applications when using softdevice.

Only Timer0 has 32/24/16 bit mode and its access is restricted when using the softdevice. For details on which bitmodes are supporting which timers see the device product specification.

Timer0 can be used by apps in a restricted way when using Multiprotocol Timeslot API

[Stanley]

Thanks Aryan for the feedback.

Could you help confirm: 1) what the minimum timer interval of app_timer is? 2) why I can't get exact timeout with the settings in my test codes? something wrong with it?

[Susheel Nuguru]

Are you using softdevice for your application? If you are using and if you have NRF_APP_PRIORITY_HIGH, with 5ms tick it might be interfering with softdevice. I will verify to see how low I can go with the RTC1 tick when using softdevice and reply later.

[Susheel Nuguru]

I am not sure why you are getting same timeout. Did you clear the counter everytime you get the TICK? otherwise it will go round the counter overflow everytime and it won't matter what you put in CC registers, you need to have below line when you initialize your timer

NRF_TIMER2->SHORTS      = 1 << TIMER_SHORTS_COMPARE0_CLEAR_Pos;