CRC on the nrf52840

[kongstrupp]

Hello!

Regarding the CRC on the nrf52840 the specifications make it clear that the maximum length of the CRC is 3 bytes, We are working on an industrial communication protocol that we want to implement on the nrf52840. This protocol needs 4 bytes crc (crc32), is there a workaround for this? 

Thanks! 

[kongstrupp]

No I think the libraries does not support it. I am currently looking into a soft way of doing it, I need some kind of function that would help me do what is described above. Yes that is one way of doing it, I will have to look into it more

[Edvin]

I see that you created a new ticket, where a colleague of mine answered one way to measure the time. However, this will work for ms (seconds/1000), but you probably want a higher resolution.

I am not sure whether there are any libraries in Zephyr that you can use to measure µs, but you can use the TIMER peripheral.

Try running this function from the hello world sample:

void test_timer(void)
{
    uint32_t sample_1;
    uint32_t sample_2;
    // Configure and start timer:
    NRF_TIMER3->BITMODE     = TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos;
    NRF_TIMER3->MODE        = TIMER_MODE_MODE_Timer << TIMER_MODE_MODE_Pos;
    NRF_TIMER3->PRESCALER   = 0;
    NRF_TIMER3->TASKS_START = 1;

    NRF_TIMER3->TASKS_CAPTURE[0] = 1;   // Take first timestamp
    k_sleep(K_MSEC(1000));              // Do something that takes time
    NRF_TIMER3->TASKS_CAPTURE[1] = 1;   // Take second timestamp

    sample_1 = NRF_TIMER3->CC[0];       // Read out first timestamp
    sample_2 = NRF_TIMER3->CC[1];       // Read out second timestamp

    uint32_t diff_time = sample_2 - sample_1;   // Calculate diff

    LOG_INF("test_timer used %d ticks.", diff_time);    // Note that clock ticks at 16 000 000 Hz
}

Just copy this function and call it from main.c. 

As I wrote in the last comment in this snippet, note that the TIMER is running at 16MHz, so 1 second = 16000000 ticks, which means that 1µs would be 16 ticks. This is the highest resolution you can get with a timer onboard the nRF52840.

BR,

Edvin

[kongstrupp]

Thanks a lot Edvin!!! 

This is great, When doing the crc32_compute I got 3 ticks which is less than 1 µs, do you think this is legit? It seems really really fast.

If its legit it means that the algorithm is really fast in itself, but then it becomes a question if it could be used on received packets in the radio an how long that takes. 

Just posting the code that I used:

uint32_t crc32_compute(uint8_t const * p_data, uint32_t size, uint32_t const * p_crc)

{

    uint32_t crc;

    crc = (p_crc == NULL) ? 0xFFFFFFFF : ~(*p_crc);

    for (uint32_t i = 0; i < size; i++)

    {

        crc = crc ^ p_data[i];

        for (uint32_t j = 8; j > 0; j--)

        {

            crc = (crc >> 1) ^ (0xEDB88320 & -(crc & 1));

        }

    }

    return ~crc;

}

void test_timer(void)

{

    uint32_t sample_1;

    uint32_t sample_2;

    // Configure and start timer:

    NRF_TIMER3->BITMODE     = TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos;

    NRF_TIMER3->MODE        = TIMER_MODE_MODE_Timer << TIMER_MODE_MODE_Pos;

    NRF_TIMER3->PRESCALER   = 0;

    NRF_TIMER3->TASKS_START = 1;

    uint8_t packetCRC[43];

    for (uint8_t i = 0; i < sizeof(packetCRC) - 1; i++) {

        packetCRC[i + 1] = 0xCC;

    }

    NRF_TIMER3->TASKS_CAPTURE[0] = 1;   // Take first timestamp

    crc32_compute(packetCRC,sizeof(packetCRC),NULL); // Do something that takes time

    NRF_TIMER3->TASKS_CAPTURE[1] = 1;   // Take second timestamp

    sample_1 = NRF_TIMER3->CC[0];       // Read out first timestamp

    sample_2 = NRF_TIMER3->CC[1];       // Read out second timestamp

    uint32_t diff_time = sample_2 - sample_1;   // Calculate diff

    printk("test_timer used %d ticks.", diff_time);    // Note that clock ticks at 16 000 000 Hz

}

void main(void)

{

test_timer();

}