app_usbd_cdc_acm_write 64 Byte Increments Issue

[jsample]

I've noticed that if I call app_usbd_cdc_acm_write with the length set to any multiple of 64 the data will not show up on my PC until I do another write with a value that is not a multiple of 64.  I get an APP_USBD_CDC_ACM_USER_EVT_TX_DONE event after the write and I can do multiple 64 byte writes in a row followed by a not 64 byte write and then all of the data will show up.

[ovrebekk]

Hi John

Is this based on the standard CDC/ACM example in SDK v15?

I will report your findings to the USB developers and see if they have some input. 

Best regards
Torbjørn

[jsample]

Hi Torbjørn,

I was working on a custom app using the cdc/acm, but I just went back to the example in SDKv15 and was able to reproduce it there after modifying it to send a 64 byte string with each button press.  One interesting thing to note - after sending 64 consecutive 64 byte strings then all 4096 bytes would show up on the PC at once.

John

[Turbo J]

Expected behaviour for bulk endpoints, as USB spec mandates that "full" consecutive packets belong to ONE transaction, and in windows an USB urb is 4KB.

If you wanted to "finish" such a transaction early, you need to send a zero packet.