How to transfer data through EPOUT2 in usbd-custom-hid-ep example

[425Ning]

Hi,

The example that I use is from here.

I have to use the USB interface to send and receive data between the host PC and the nRF52840 chip.

In the example, I can only send the data from the chip to the computer through EPIN2, but can not send the data from the computer to the chip through EPOUT2.

 • send the data from the chip to the computer

   Press Button 2 → send the data through EPIN2 → SUCCESS (the array that the chip sent shows on the Python terminal, and "transfer complete" shows on PUTTY)

    

 • send the data from the computer to the chip

   Input the data (I use RTT Viewer to send the data. But when I input the string, it shows the warning messages.) → FAIL

      

What should I do to let the data transfer successfully?

What function should I add to call the NRF_DRV_USBD_EVT_EPTRANSFER event to let the receive_custom_data() function work? 

static void usbd_event_handler(nrf_drv_usbd_evt_t const * const p_event)
{
    switch (p_event->type)
    {
    ...
    case NRF_DRV_USBD_EVT_EPTRANSFER:
        if (NRF_DRV_USBD_EPIN2 == p_event->data.eptransfer.ep)
        {
            NRF_LOG_INFO("EP2IN transfer complete");
        }
        else if (NRF_DRV_USBD_EPOUT2 == p_event->data.eptransfer.ep)
        {
            NRF_LOG_INFO("EP2OUT transfer complete");
            receive_custom_data();
        }
        ...
        break;
    ...
}

[ovrebekk]

Hi Ning

You should be able to get a higher speed if you use a different USB class, such as CDC or MSC. 

We have done some testing internally to evaluate the transfer speed using these classes, and got the following results:

MSC write zeros to an empty drive (Linux): 513 kB/s (4104 kbit/s)

CDC ACM binary file transfer (Linux): 323 kB/s (2584 kbit/s)
CDC ACM binary file transfer (Windows): 215 kB/s (1720 kbit/s)

I don't know where the 8Mbps number comes from, but keep in mind that if you use an isochronous endpoint you are not guaranteed to have all the data come through, and you would have to accept a certain amount of packet loss. 

Best regards
Torbjørn

[425Ning]

Hi Torbjørn,

ovrebekk said:

You should be able to get a higher speed if you use a different USB class, such as CDC or MSC. 

I'd used CDC and PySerial(in Python) before. But I want to transfer the data through the endpoint directly but not through the virtual COM port. So I tried to use HID and PyUSB(in Python) to implement my project.

ovrebekk said:

if you use an isochronous endpoint you are not guaranteed to have all the data come through, and you would have to accept a certain amount of packet loss. 

If reliability is not considered, is it possible to use an isochronous endpoint to get a higher data rate than CDC?

Best regards,

Ning

[ovrebekk]

Hi Ning

425Ning said:

If reliability is not considered, is it possible to use an isochronous endpoint to get a higher data rate than CDC?

Isochronous endpoints essentially give you a guaranteed and stable data rate, but without the guarantee that all the packets are received. The main use case for this is audio or video streaming where it is more important to keep latency low and throughput stable, rather than make sure that all the data is successfully transmitted. 

Our examples only use isochronous endpoints for CD quality audio, so we haven't really tried to push the data rates to the limit. 

Best regards
Torbjørn