A note to self about using the CircuitPython native USB for serial data with usb_cdc

using_circuitpython_native_used_for_serial_data.md

Using the CircuitPython native USB for serial data

I was trying to get some CP code to talk to Processing over a serial port interface. Of course, the CP REPL is normally available on whatever serial port the board mounts by default. This is a little complicated, because although you can use something like print from the REPL (or code.py) to print a value to the console, sys.stdin and sys.stdout are text-based streams, which may not be what you want. And you'll get REPL noise to boot. Not ideal.

This is where usb_cdc comes in. It's a handy library for managing the USB CDC (serial) on most CircuitPython boards. First, usb_cdc can be used to control how many serial ports CP will provide at startup. You can modify boot.py to make this work:

import usb_cdc

usb_cdc.enable(console=True, data=False)

This is equivalent to the standard setup. When you plug in your CP device, you'll get one serial port, which will have the REPL on it, and is mapped to sys.stdin and sys.stdout, all of which goes through the console object, which is a binary stream. This means you can do stuff like this in your code.py:

import usb_cdc

console = usb_cdc.console
console.write(bytes([1]))

This is great, since it means you can send binary data, which is useful if you don't want to muck with the REPL's textstream. However, you're still using the same serial port. We can do better. If you put this in boot.py, you'll get a second serial port when you plug in your board:

import usb_cdc

usb_cdc.enable(console=True, data=True)

Now you can connect to the second serial port and avoid the REPL noise. You can do this with the usb_cdc.data binary stream, in either the REPL or from code.py:

import usb_cdc

console = usb_cdc.data
console.write(bytes([1]))

For reference, take a look at the documentation for the usb_cdc library and this handy guide to configuring the USB ports in CircuitPython.

Checking on CircuitPython serial ports

There is a handy utility library, Adafruit_Board_Toolkit, that provides some utilities for inspecting the serial ports on a system to see if they're avaliable from CP and if so, if they console or data reports.

Once you've installed this library in Python, it's pretty easy to use. For example, to list all of the likely CP serial ports:

from adafruit_board_toolkit import circuitpython_serial as cps

print([port.device for port in cps.comports()])

@stonehippo,

Thank you for your reply.

The GPS receiver I am trying to use only has a USB connector.
In looking at the available functions in the CircuitPython versions of usb_cdc for the RP2350 it shows the following:
`>>> import usb_cdc

dir(usb_cdc)
['class', 'name', 'Serial', 'dict', 'console', 'data', 'disable', 'enable']

dir(usb_cdc.console)
['class', 'next', 'read', 'readinto', 'readline', 'write', 'connected', 'flush', 'in_waiting', 'out_waiting', 'readlines', 'reset_input_buffer', 'reset_output_buffer', 'timeout', 'write_timeout']

dir(usb_cdc.data)
['class']
`
With read, readinto, readline, and readlines I assumed what I want to do is supported. Oh, well.

Bruce

@b-blake yes, usb_cdc has the sort of methods you need, but it's intended for presenting a serial port to a USB host device, e.g., a laptop or desktop PC. So it's not quite what you need.

What you need is usb.core.Device. Specifically usb.core.Device.read(), which is the right way to get data from a USB device when the CircuitPython device is the USB Host.

Thank you very much.