USBD CDC seems too slow

RE: USBD CDC seems too slow

ovrebekk — Thu, 26 Nov 2020 12:44:05 GMT

I don't know if this is really answering the question, but in order to figure out a good way to do buffering of the incoming CDC data I made a small example that uses the nrf_queue library to organize a 'fifo of buffers', each 64 bytes in size.

The trick is to pre-load two buffers for the CDC driver when the APP_USBD_CDC_ACM_USER_EVT_PORT_OPEN event occurs, and then provide one more buffer every time the APP_USBD_CDC_ACM_USER_EVT_RX_DONE event occurs.

Then you ensure that the CDC driver will always have a buffer in spare, in addition to the buffer currently being written to, so that you don't get any data slow downs caused by application processing.

You can find my code example here:
https://github.com/too1/usbd-ble-uart-c/tree/throughput_testing

It only writes the incoming data to the log, but this can be replaced with BLE transfers quite easily.

Best regards
Torbjørn

RE: USBD CDC seems too slow

Ros — Wed, 25 Nov 2020 23:22:42 GMT

I am using Realterm with Winows10 to send data from a file of 240 bytes long.

Could you please answer the second question?

RE: USBD CDC seems too slow

ovrebekk — Wed, 25 Nov 2020 09:01:39 GMT

I will try to do some testing on my end to see if I can reproduce the issue you are seeing. 240 bytes in more than 50ms sounds very slow indeed.

How are you sending the data from the host side?

Are you using some standard terminal software, or a custom application?

Best regards
Torbjørn

RE: USBD CDC seems too slow

Ros — Tue, 24 Nov 2020 09:49:49 GMT

Hi,

Thanks for the reply. Yes, I am trying to get higher data rate for receiving part. I didn't show whole code here, the idea is:

1. The first transfer is setup on APP_USBD_CDC_ACM_USER_EVT_PORT_OPEN event

case APP_USBD_CDC_ACM_USER_EVT_PORT_OPEN:
        {
            /*Set up the first transfer*/
            ret_code_t ret = app_usbd_cdc_acm_read_any(&m_app_cdc_acm,
                                                   rx_bytes,
                                                   READ_SIZE);
        }

This is the reason for incrementing rx_buffer pointer as I am trying to setup "app_usbd_cdc_acm_read_any" for next transaction.

2. For each "APP_USBD_CDC_ACM_USER_EVT_RX_DONE" event, set buffer for next transaction and clear timer. The code is posted in my first post.

3. On timer timeout event, sends the received data through BLE, and set the buffer pointer for "app_usbd_cdc_acm_read_any" to the beginning.

static void timer1_event_handler(nrf_timer_event_t event_type, void * p_context)
{
    // Copy the data and initialize BLE data transmission
    uint16_t len = get_rx_data_len();
    reset_rx_buf_ptr();
    
    memcpy(ble_buffer, rx_buffer, len);
    ble_nus_c_send_data(ble_buffer, len);
    
    /* Set buffer pointer to the beginning */
    ret_code_t ret = app_usbd_cdc_acm_read_any(&m_app_cdc_acm,
                                                   rx_buffer,
                                                   READ_SIZE);
}

In my testing, the function call to reset the buffer pointer to the beginning is not working. The next data write to the location set by the step 2.

RE: USBD CDC seems too slow

ovrebekk — Tue, 24 Nov 2020 09:30:25 GMT

Hi Ras

It should definitely be possible to get higher data throughput than what you are experiencing.

To be clear, is the problem receiving data from the USB host, or sending it?
The code you attach is showing the receiving part.

Also, why are you incrementing the rx_buffer before receiving the data?
In the original example the rx_buffer pointer is pointing to an empty buffer, and there is no need to increment this buffer until after the data has been transferred into it.

Best regards
Torbjørn