USB CDC ACM data transfer

[Petar02]

Hello,

I have a problem with a tablet that i was planning on using (Xiaomi Pad Pro), my device (nrf25840 dk) is sending data in packets, 4000bytes per packet and its sending 77 packets per second. And its working for every other mobile device including some xiaomi s, but for this tablet somehow the speed is 2 times slower. on the nrf side, the data is processed at the set speed but when data needs to be send its 2 times slower that set speed, so there is no missing data on tablet side. My question is there a way to force on the nrf side to set a seed, because its seems that tablet slows down communication with nrf and data transfer.

Thanks in advence.

Petar.

[Kenneth]

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

[Petar02]

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.

[Kenneth]

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

[Petar02]

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.