nrf5340 transfer data with UART and CDC_ACM, know when to add data to UART buffer

[stevenminnick]

I want to transfer a bunch of data over the virtual com port to a PC using the USB port on the nrf5340.  I'm using the CDC ACM sample: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/samples/subsys/usb/cdc_acm/README.html

That sample takes a character in and pushes the character back out.  Simple and it works. 

Since I want to dump a bunch of data, I started doing the same uart_fifo_fill() command.  It appears I can only add 1024 bytes to that at a time, which is fine.  That function returns the number of bytes it send (or added to the buffer). 

My question is, how do I know when I can add more data to that buffer?  When can I call that command again.  There were a couple other functions in UART.H and the one when TX was completed is not implemented and the other is "TX buffer can take at least 1 byte of data" but that doesn't seem ideal.  I want to know when I can send 1024 bytes of data again.  

What's the ideal method for pushing a bunch of data through the UART so that it runs as fast as possible and I know when I can push more data into the buffer?

[stevenminnick]

I don't believe the source buffer is modified by anything else.  It's a single threaded application.  And it's created right before sending.  Unless I have a crazy weird memory corruption bug somewhere.  But the fact that when I send less bytes than the max to the UART with fifo fill, it always works.  And when I don't it always fails.  Same code just calling fifo fill with more than it can take and no wait between calls. 

[Einar Thorsrud]

I see, then I don't know. Are you able to make a minimal example that demonstrate this issue and runs on a DK? If so I can test on my end.