RE: doubt with saadc example from SDK

andribiotico — Thu, 24 Nov 2016 11:19:03 GMT

thnak you!!

RE: doubt with saadc example from SDK

Jørgen Holmefjord — Wed, 16 Nov 2016 13:42:31 GMT

Hi,

The default frequency of TIMER0 is 16 MHz. If you have not changed this, the return value of nrf_drv_timer_ms_to_ticks() will overflow the uint32_t return variable.

The function calculates the number of ticks using the formula:

(time_ms * 16000 kHz) / (2^prescaler)

where prescaler is related to frequency like this:

f = 16000 kHz / 2^prescaler

With the timer frequency set to 16 MHz, this will give a prescaler value of 0. For 5 minutes, the formula then return:

4800000000 > 4294967295

This overflow of the uint32_t variable will give a return value from nrf_drv_timer_ms_to_ticks() of 505032704. There is a ASSERT check within the function to avoid this overflow, but this does not seem to fire. I will check this out.

To run timers for longer time than ~4.48 minutes, you will have to set a lower frequency of the timer. You can do this when configuring your timer with the following lines:

nrf_drv_timer_config_t m_timer_config= NRF_DRV_TIMER_DEFAULT_CONFIG(0);
m_timer_config.frequency = NRF_TIMER_FREQ_8MHz;
err_code = nrf_drv_timer_init(&m_timer, &m_timer_config, timer_handler);

Or you can set the prescaler value directly:

NRF_TIMER0->PRESCALER = 1;

Notice that you cannot use the nrf_drv_timer_ms_to_ticks() function for values above (2^32)/16000 ~= 268435ms. You can still do the calculation yourself, or create your own function to do this calculation for larger values.

Best regards,

Jørgen