How to accurately measure active USB current on nRF52840?

[catsunami]

I am using nRF52840 Preview DK (0.12.0) and I want to measure USB currents. I am using the DK as a starting point for doing measurements on custom HW and verifying that we get the same measurements there.

I used the usbd_cdc_acm example from the SDK (15.0.0) as a starting point. My main function is:

   

int main(void)
{
    NRF_P0->DIRSET = 0xFFFFFFFC;
    NRF_P0->OUTCLR = 0xFFFFFFFC;
    NRF_P1->DIRSET = 0xFFFFFFFF;
    NRF_P1->OUTCLR = 0xFFFFFFFF;
   
    ret_code_t ret;
    static const app_usbd_config_t usbd_config = {
        .ev_state_proc = usbd_user_ev_handler
    };

    ret = nrf_drv_clock_init();
    APP_ERROR_CHECK(ret);
    
    app_usbd_serial_num_generate();

    ret = app_usbd_init(&usbd_config);
    APP_ERROR_CHECK(ret);

    app_usbd_class_inst_t const * class_cdc_acm = app_usbd_cdc_acm_class_inst_get(&m_app_cdc_acm);
    ret = app_usbd_class_append(class_cdc_acm);
    APP_ERROR_CHECK(ret);

    ret = app_usbd_power_events_enable();
  
    size_t size = sprintf(m_tx_buffer, "a");
    
    while (true)
    {
       while (app_usbd_event_queue_process())
        {
            // Nothing to do
        }
        
        app_usbd_cdc_acm_write(&m_app_cdc_acm, m_tx_buffer, size);
    }
}

All it does is spew 'a' as fast as possible (I think). However, the measurement I get swings between 6.8mA and 7.9mA. I think on average it matches what the spec suggests, but I was wondering if there is a better way to measure this active current?

[Simonr]

Hi

I am sorry for the initial misunderstanding. However I still would recommend using the PPK for a more accurate measruement. What DMM are you using?

As for your code I'd say it looks good, but you can try sending bigger packets than just "a". As you would use less time executing the app_usb_cdc_acm_write() function. 

Best regards,

Simon

[catsunami]

Thanks Simon. I'm using a Keysight 34465A DMM, I think it should be accurate enough. I'll try longer packets and let you know what happens.