Serial (CDC) module for USB Host #10283
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This is a module wrapping TinyUSB's
class/cdc/cdc_hostlibrary into a pyserial-like API so that a CircuitPython board with a USB Host can talk to other devices with CDC interfaces over USB.It is possible to talk CDC using only the
usb.core.Device.read/.writemethods. However, this only works in blocking mode. I tried setting up asynchronous communication by supplying a timeout to.read(), but when I tested my code I missed data.As I understand, CDC reads are performed by sending an
INpacket, then waiting for the device to eventually send aDATApacket back. This transaction cannot be cancelled halfway through, so even if CircuitPython times out and moves onto other things, the data transfer is going to happen regardless.I think the only way to build an asynchronous API around sending data through the
usb.corelibrary is to use FIFOs so that even if CircuitPython times out and moves on, the data is still appended to the FIFO and can be read back later. This is how TinyUSB's Endpoint Stream API works. Theclass/cdc/cdc_hostlibrary I am wrapping internally uses Endpoint Streams for the rx and tx endpoints.I was debating building a module around the cdc host or endpoint streams and decided against endpoint streams since they are only exposed in TinyUSB's private headers. They don't require a significant amount of code to implement though, so if you think that it is better to build a lower-level library than a higher-level library I could submit that as a PR instead. I have most of the code written, but it does not work yet.