USBD with CDC ACM class Windows 10 maximum data rate

[Sebastian]

Hello All,

I am a bit confused with the maximum speed that is achievable with the USBD CDC ACM driver. 

In Previous threads like this:

https://devzone.nordicsemi.com/f/nordic-q-a/23754/which-usb-class-should-i-use-for-maximum-data-rates-on-nrf52840-and-what-is-the-maximum-size-of-transfer-if-i-use-cdc_acm

Nordic employees state that 2Mbps (250kbytes/sec) is achievable with the driver. 

In a different thread here:

https://devzone.nordicsemi.com/f/nordic-q-a/23265/max-frame-size-usb-bulk-transmision-increase-speed

when user stevenB claims to reach 2.5Mbps, bjorn-spockeli claims that reachable speed is 700kbytes/sec (5.6Mbps) is possible with MSC.

I really doubt that 2 and 2.5Mbps are actually achievable with the CDC ACM driver provided in SDK 15.2.0. I am more inclined to accept something around 500kbps (62.5kbytes/sec) for the CDC ACM.

Could someone @nordic clarify what is actually achievable and perhaps provide some guidance of how to get rates higher than a 1 Mbps (if at all possible with CDC ACM)?

Thank you for your answers.

[ovrebekk]

Hi Vasileios

I have checked with the team, and they agreed to redo the tests for the latest SDK. 

I don't know how quickly the results will be ready, but as soon as they are I will update this case with their findings. 

Best regards
Torbjørn

[Sebastian]

Thanks ovrebekk

[ovrebekk]

Hi

The team was quick to respond, and here are their results:

MSC write zeros to an empty drive (Linux): 513 kB/s (4104 kbit/s)

CDC ACM binary file transfer (Windows): 215 kB/s (1720 kbit/s)

Best regards
Torbjørn

[Sebastian]

Hello ovrebekk,

Thanks for the quick answer. I have run my own experiment and can confirm this number now. 

Is there an explanation about this limit? Is it a matter of the Windows driver (usbser.sys) or a limitation imposed by the USB protocol operated as CDC?

I mean what happens if one keeps loading the 'app_usbd_cdc_acm_write' routine with more and more bytes by increasing the 'length' function parameter?

Thank you for your answers.

[ovrebekk]

Hi Vasileios

This is a limitation by Windows, yes. On Linux we see higher data rates. 

Best regards
Torbjørn