How to get the voltage level of USB D+ and D-

Question

Hi, I want to distinguish whether the USB is connected to a PC or a power adapter. 
 For my application, FATFS still needs to work when connected to a power adapter, so it can not be distinguished via the USB event, as described in https://devzone.nordicsemi.com/f/nordic-q-a/36414/usb . 
 My idea is that distinguish by the voltage level of D+ and D- when USB is connected. I found a USB register DPDMVALUE relate to the D+ and D-, but I always get a zero value whether connecting to a PC or a power adapter. 
 My question is how to get the value of DPDMVALUE ? Does it need any other setting? Thanks!

Jason · Accepted Answer

Hi Dmitry.

Thank you for your reply.

For the wall adapter, APP_USBD_EVT_POWER_DETECTED is not the only event.

void usbd_user_ev_handler(app_usbd_event_type_t event)
{
	if(event)
	{
		NRF_LOG_INFO("USB event:%d", event);
	}
    switch (event)
    {
        case APP_USBD_EVT_DRV_SUSPEND:
            //bsp_board_led_off(LED_USB_RESUME);
            break;
        case APP_USBD_EVT_DRV_RESUME:
            //bsp_board_led_on(LED_USB_RESUME);
            break;
        case APP_USBD_EVT_STARTED:
            //bsp_board_led_on(LED_USB_START);
            break;
        case APP_USBD_EVT_STOPPED:	  
		    NRF_LOG_INFO("USB stopped");
            UNUSED_RETURN_VALUE(fatfs_init());
            app_usbd_disable();
            //bsp_board_leds_off();
            break;
        case APP_USBD_EVT_POWER_DETECTED:
            NRF_LOG_INFO("USB power detected");
            if (!nrf_drv_usbd_is_enabled())
            {
                fatfs_uninit();
                app_usbd_enable();
            }
            break;
        case APP_USBD_EVT_POWER_REMOVED:
            NRF_LOG_INFO("USB power removed");
            app_usbd_stop();
            m_usb_connected = false;
            break;
        case APP_USBD_EVT_POWER_READY:
            NRF_LOG_INFO("USB ready");
            app_usbd_start();
            m_usb_connected = true;
            break;
        default:
            break;
    }
}

the log as below when wall adapter is connected:

00> <info> app: USB event:8
00>
00> <info> app: USB power detected
00>
00> <info> app:

00>
00> <info> app: Un-initializing disk 0 (QSPI)...
00>
00> <info> app: USB event:10
00>
00> <info> app: USB ready
00>
00> <info> app: USB event:15
00>
00> <info> app: USB event:13
00>
00> <info> app: USB event:2

the log as below when PC is connected:

00> <info> app: Un-initializing disk 0 (QSPI)...
00>
00> <info> app: USB event:10
00>
00> <info> app: USB ready
00>
00> <info> app: USB event:15
00>
00> <info> app: USB event:13
00>
00> <info> app: USB event:2
00>
00> <info> app: USB event:3
00>
00> <info> app: USB event:1
00>
00> <info> app: USB event:15
00>
00> <info> app: USB event:1
00>
00> <info> app: USB event:15
00>
00> <info> app: USB event:15

I tried to move the fatfs_uninit function to the case of APP_USBD_EVT_DRV_RESUME event, but the MSC did not work .

Actually, My target is that do not un-initialized the disk if connect to a wall adapter.

How to get the voltage level of USB D+ and D-

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