Random number generator not so random

[T IJ]

Hello DevZone,

SDK 15.0.0, sd132

I'm working on the random number generator peripheral in the nRF52832 and used the peripheral example in my SDK.

When I upload the example code to my devkit I can clearly see that every time the microcontroller boots the numbers differ from each other.

When I want to implement that feature in my project, using the same lines of code and files and settings (I believe) it always seems to give me the same number.

Even across multiple devkits. 

So can anyone tell me what I'm doing wrong here?

Kind regards

[snoopy20]

First you're not explaining yourself correctly. You state when you upload to 'my devkit' you get a random number, then you later say you don;t get random across 'multiple devkits'. Try explaining what a devkit is, what your 'project' is (custom prototype board?)...

[T IJ]

I have multiple standard PCA10040 devkits 

[T IJ]

After roughly resetting the device 20 times.

I've added my code below, it is an exact copy from the example except that I only need 6 random numbers instead of the 16 in the example.

uint8_t                 available;
uint8_t                 p_buff[6];
nrf_drv_rng_config_t    config;

config.error_correction = 1;
config.interrupt_priority = 7;

err_code = nrf_drv_rng_init(&config);
APP_ERROR_CHECK(err_code);

nrf_drv_rng_bytes_available(&available);
uint8_t length = MIN(6, available);

err_code = nrf_drv_rng_rand(p_buff, length);
APP_ERROR_CHECK(err_code);

[snoopy20]

OK. So this is ONE nRF52-DK you are using AND you receive SIX identical numbers. YES/NO?

[T IJ]

Yes

[T IJ]

If I use the following own witten code, the numbers are random every time.

NRF_RNG->CONFIG = RNG_CONFIG_DERCEN_Msk;

NRF_RNG->TASKS_START = 1;

uint8_t uReady = NRF_RNG->EVENTS_VALRDY;
while(uReady == 0)
{
    uReady = NRF_RNG->EVENTS_VALRDY;
}

NRF_RNG->TASKS_STOP = 1;

NRF_LOG_INFO("%d",NRF_RNG->VALUE);

[snoopy20]

I haven't used this peripheral but I prefer writing home-brew rather than the abstracted libraries in most cases as you can tell what's going on. Some things come to mind, the obvious one is the library probably uses interrupts and you are not. Second is the RND is probably used by SoftDevice (I guess) for the encrypted connection so maybe the nrf library is calling it and you have a problem there.

If you're doing this on boot before softdevice init and without need to refresh the numbers I'd be happy with your approach, with a loop for however many you need.

[T IJ]

Well thanks for your help at least, I've figured out that the softdevice indeed uses this peripheral and makes it unusable for me. I think I'll do all my random number generation prior to starting the softdevice

[snoopy20]

That's what I'd do. You can probably call the softdevice to try and share the generation but doing it before init is cleaner imo. It comes down to whether you need new numbers later on.

[T IJ]

Yea you're probably right, I've found this that works with the softdevice.
The only thing I dislike about this is waiting for the softdevice to generate a number, but its something I have to deal with.

for(uint8_t i = 0; i < 6; i++)
{
    while(len == 0)
    {
        APP_ERROR_CHECK(sd_rand_application_bytes_available_get(&len));
    }
    APP_ERROR_CHECK(sd_rand_application_vector_get(&p_buff[i], len));
    len = 0;
}