USB CDC ACM float support / corrupted messages

[mxhll]

Hello,

I'm using sprintf to prepare a string to be sent with app_usbd_cdc_acm_write() with a usb configuration resembling your usbd_cdc_acm example of SDK 17.0.2.

When floating point support for printf (&sprintf) is enabled in SES, the message appearing on the virtual com port on a windows machine is unreadable.

Is this expected behaviour? It took me quite a while to figure this out and I havent found anything in the documentation in this regard.

Please let me know if there is a workaround, as I want to use floating point support in printf.

Thank you

(nRF52840, SDK 17.0.2, SES R 5.30)

Edit: To clarify: As long as float support is enabled, pp_usbd_cdc_acm_write() will only transmit corrupted messages, regardless of actual floating point content. Shouldnt even matter as it only receives a pointer to a char array and the floating point printing is handled by sprintf, which is working just fine by itself.

Edit2: Now it just works. Have not changed anything and I'm even more clueless why that happened, as it was repeatable many times before. So probably an issue of the terminal application in windows.

[Jørgen Holmefjord]

Hi,

Could it be that the variable you pass a pointer to in app_usbd_cdc_acm_write() goes out of scope and is overwritten by some other part of the application, before the USBD library is able to fully process and transmit the content?

Best regards,
Jørgen

[mxhll]

Hello Jørgen, this is very much possible. Do you by chance know if there is an example use case of a fifo or ring buffer to avoid overwriting cdc acm transfers? If I understand it correctly the usbd class uses some form of fifo, but not the cdc acm implementation.  I think it is a bit challenging to use for the lesser experienced developers.

Thank you for your answer, I will mark this as solved

[Jørgen Holmefjord]

You could just declare the buffer static or global, which should prevent it from being overwritten.

Alternatively, you can use the Ring buffer library from the SDK.