USB CDC ACM data transfer

RE: USB CDC ACM data transfer

Petar02 — Tue, 27 Jan 2026 09:19:41 GMT

Hi,

I didn't actually try to reduce the number of packets, because my adc runs at 153kHz and I just collect data from the adc output into ping pong buffers and send them using the nrf USB CDC ACM function to send data. sdk-zephyr/samples/subsys/usb/cdc_acm/src/main.c at v4.0.99-ncs1-2 · nrfconnect/sdk-zephyr. It goes on its own down to 45 packets just for that one tablet haha, but stil adc running on 153kHz.

RE: USB CDC ACM data transfer

Kenneth — Tue, 27 Jan 2026 09:02:38 GMT

Hi,

As a USB device I don't think it's much that can be done no, I am still a bit interested in how this 77 packet/s is implemented, so just to make sure I don't misunderstand: If you keep 4000bytes, but slowly reduce from 77 packets/s (308kBps) to e.g. 70 (280kBps) or 60 (240kBps), does that work, or you have to go all the way down to 45 packet/s (180kBps).

Kenneth

RE: USB CDC ACM data transfer

Petar02 — Mon, 26 Jan 2026 14:24:18 GMT

Hi Kenneth,

I have tried to increase a packet size to 8000bytes and 12000bytes and for every mobile device and now for other tablet its working fine, it manages to reach a speed of 308kB/s. But only for that one tablet max speed is 180kB/s, its like they went in USB slow device. So i was wondering is there a way for nrf to force full speed USB on the host.

RE: USB CDC ACM data transfer

Kenneth — Mon, 26 Jan 2026 14:11:42 GMT

Hello,

I don't think you are far off the realistic throughput for device->host over USB for a USB full-speed device. It seems you are at the moment pushing about 308kB/s.

Have you tried to reduce the 77 packet/s downwards until it can catch up? E.g. try with 70 packets/s or 60 packets/s for test.

Kenneth