USB endpoint transmitter returns busy

[Amirtk]

Hi,

I have a little program that is built on top of the USB CDC ACM example. This little program waits for an SPI event, when the event arrives, it reads the SPI peripheral and sends out the data over USB. SPI events frequency (USB send frequency) is 850 Hz. At every transfer only one packet of size 50B is sent. This is almost all this program does.

On the host side, I noticed that almost 70 packets are not received in every 10000 packets. I realized that the packets that are not received, have never been sent out since the nrfx_usbd_ep_transfer function in app_usbd_cdc_acm_write returned NRF_ERROR_BUSY in those cases. The NRF_ERROR_BUSY occurances does not have a regular pattern, they could happen at different times and in different numbers.

Could you please help me understand what could be causing this and how I could resolve it? Is 850 Hz to high a frequency? Could it be that the chip is in low-power mode? Could it be that the DMA is not freed by the SPI at these times?

I am using nRF52840 Dongle and nRF5_SDK_15.3.0_59ac345.

Kind regards,

Amir

[Kenneth]

Hi,

I believe the problem here may be that the USB endpoint is setup to use bulk transfer and not interrupt. You can read more about bulk vs interrupts transfer here:
https://www.beyondlogic.org/usbnutshell/usb4.shtml#Interrupt
https://www.beyondlogic.org/usbnutshell/usb4.shtml#Bulk

In specific check out: "Bulk transfers will use spare un-allocated bandwidth on the bus after all other transactions have been allocated. If the bus is busy with isochronous and/or interrupt then bulk data may slowly trickle over the bus. As a result Bulk transfers should only be used for time insensitive communication as there is no guarantee of latency."

In other words: BULK transfer will overall have higher throughput, but may have longer latency for individual packets. So in this case you will have to handle the case that NRF_ERROR_BUSY may occur. For instance buffer the data until they are sent successfully.

Best regards,
Kenneth

[Amirtk]

Hi,

When I look at the output of the lsusb -t command, I see that the dongle is recognized as a 12Mb/s device. I measured the traffic on the bus using Wireshark and it added up to only 277Kb/s and the bus is free of isochronous or interrupt transfers. So it seems there is plenty of bandwidth. Can you please help me understand why the transfer function returns NRF_ERROR_BUSY?

Kind regards,

Amir

[Amirtk]

Just to add that I even reduced frequency from 850Hz to 200Hz and the problem is less frequent but still there.