nrf52 timer example, over 5 minutes event handler.

[JW]

When I test the timer example with nrf52832 DK, it so nice to 4 minutes.

uint32_t time_ms = 240000;

But when I set the time interval over 5 minutes, then the event handler run more rapidly.

So, what should I do for long time period?

[Jørgen Holmefjord]

Hi JW,

By default, the timer runs at 16 MHz and the maximum bit width of the timer is 32-bit. This corresponds to a maximum timer value of:

(2^32)-1 = 4294967295

The reason why you see event handler more rapidly with 5 minutes, is that nrf_drv_timer_ms_to_ticks use the configured frequency and the time_ms parameter to generate the number of ticks, and return this in a 32 bit variable:

5*60*16000000 = 4800000000

This overflows the 32 bit variable, giving you a tick count of 505032705 = ~31.5 seconds.

You need to change the frequency of the timer to make it run run slower. This is done by changing the timer configuration:

timer_cfg.frequency = NRF_TIMER_FREQ_8MHz;

You can set it to any of the frequencies documented here.

Best regards,

Jørgen

[JW]

Hello Jørgen,

Thank you for answering my question. I can understand the timer configuration by your explanation.

But when I changed the frequency on 'nrf_drv_config.h'

from 16MHz

#define TIMER0_ENABLED 1

#if (TIMER0_ENABLED == 1)
#define TIMER0_CONFIG_FREQUENCY    NRF_TIMER_FREQ_16MHz
#define TIMER0_CONFIG_MODE         TIMER_MODE_MODE_Timer
#define TIMER0_CONFIG_BIT_WIDTH    TIMER_BITMODE_BITMODE_32Bit
#define TIMER0_CONFIG_IRQ_PRIORITY APP_IRQ_PRIORITY_LOW

#define TIMER0_INSTANCE_INDEX      0
#endif

to 8MHz

#define TIMER0_CONFIG_FREQUENCY    NRF_TIMER_FREQ_8MHz

and set the timer interval 1 second

uint32_t time_ms = 1000;

nothing changed.

The timer event handler run every 1 second at both configuration.

[Jørgen Holmefjord]

Your observations are correct, this change will not affect the time between event handler calls. What it will change is the output from nrf_drv_timer_ms_to_ticks(), the variable time_ticks. With time_ms = 1000 and frequency set to 16 MHz, the returned value will be 16 000 000. With time_ms = 1000 and frequency set to 8 MHz, it will return 8 000 000. Reducing the frequency will reduce the resolution of the timer, but it will increase the time the timer can run before overflowing.

[JW]

Well, but when I set the time interval 5 minutes with time_ms = 300000 after change frequency to 8MHz, the event handler also run every 30 seconds.

[Jørgen Holmefjord]

I see there is a 32 bit multiplication inside the function, resulting in the return value being wrong. If you change line 618 in nrf_timer.h from return ((time_ms * 16000UL) >> prescaler); to return ((time_ms * 16000ULL) >> prescaler);, you should get correct behavior.