RE: USB missing byte

JoEi — Tue, 01 Mar 2022 09:55:13 GMT

Hi Hung Bui,

thanks for the fast reply.

I found the error yesterday evening and it is exactly the same error you mentioned.
It's funny. I searched for "1 byte missing usb" but didn't find the case you posted and all the other cases I found were similar but ultimately different.

Kind regards
Johannes

RE: USB missing byte

Hung Bui — Tue, 01 Mar 2022 08:52:09 GMT

Hi Johannes,

If you search for "1 byte missing usb devzone" you can find many cases on devzone asked about this. It's a bit confusing in the way the function app_usbd_cdc_acm_read() was designed.
I would suggest to have a look here.

As I explained in the case, quoted:
===========

As Dmitry already explained to you app_usbd_cdc_acm_read() doesn't read the byte(s) immediately, it only provide a buffer for the next "byte" (or bytes depends on the buffer size you provided) to come.

So in your case you can call app_usbd_cdc_acm_read() with the buffer size of 64 bytes. See APP_USBD_CDC_ACM_USER_EVT_PORT_OPEN event in the cdc example.

When you receive APP_USBD_CDC_ACM_USER_EVT_RX_DONE event this meant that there are 64 bytes already in the buffer and you can start using it. You don't need to call app_usbd_cdc_acm_read() again to read them. You call app_usbd_cdc_acm_read() again to prepare the buffer for the next 64 bytes.

And since app_usbd_cdc_acm_read() can handle double buffer, you can call app_usbd_cdc_acm_read() two times with two different buffers in APP_USBD_CDC_ACM_USER_EVT_PORT_OPEN event. This way you have double buffer and can handle if there are more than 64 bytes comes before you can process them.

============

I would need to see your full code and what inside your app_usbd_dev_read_data() but as mentioned above when you call app_usbd_cdc_acm_read() it's for the next byte to come, the current byte is already in the buffer when you receive APP_USBD_CDC_ACM_USER_EVT_RX_DONE event.