Provide an optional second USB CDC serial channel

[dhalbert]

Several other issues (#231, #3849, for example) want to communicate serially with a host computer without the REPL interfering. We have discussed providing a second USB CDC serial device. This probably needs dynamic USB descriptors (#1015), because of the shortage of endpoints, and the confusion that an always-present second CDC might present.

[emard]

I'd suggest to test-implement 2nd CDC channel with static descriptor,
observe problems what happens with host os and fix problems then.

I can test and what happens with dual CDC on linux.

[tannewt]

TinyUSB has a dual CDC example already here: https://github.com/hathach/tinyusb/tree/master/examples/device/cdc_dual_ports

[ladyada]

during discussion today i asked if we could implement secondary CDC UART on one or two platforms that have the spare endpoints now, rather than blocking on a dynamic descriptor implementation

[tannewt]

during discussion today i asked if we could implement secondary CDC UART on one or two platforms that have the spare endpoints now, rather than blocking on a dynamic descriptor implementation

We certainly could. I think that'd get us most of the way to supporting dynamic descriptors. We have most of the prerequisites in place already.

[kevinjwalters]

For reference, this may well be of interest for this user and anyone who finds this question: How to do non blocking usb serial input in circuit python?.

[ladyada]

added