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  • Author Author: Jan Cumps
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help: how do I use the Pico USB as COM port, with support for RX interrupt?

Jan Cumps

I know how to use plain UART on the Pico, with receive interrupt. See advanced_uart.c.

I also know how to make the UART show and work as a COM device. See hello_usb makefile.

I'm looking for help on the following: use the USB as a COM, and have support for incoming traffic (RX) interrupts.
Have you done that, or do you know how to do that? Please share.

I've made a test bed. It's the uart_advanced pico-example. I just copied it, and made changes to the build file.
This will allow that it's compiled, wherever the SDK sits. As long as you set this environment variable: PICO_SDK_PATH to your SDK location. For me: C:\Users\jancu\Documents\Pico\pico-sdk
Also contains a pico-probe config.

uart_advanced.zip

To switch to USB, uncomment the two last lines in the CMake file.

# enable usb output, disable uart output
# pico_enable_stdio_usb(uart_advanced_usb 1)
# pico_enable_stdio_uart(uart_advanced_usb 0)

  • current status: a compromise:

    • If the UART library is linked (#if LIB_PICO_STDIO_UART), I make my code use the interrupt version.
    • If the USB library is linked (#if LIB_PICO_STDIO_USB), I make my code use a polling task (check character, no wait if none available), and then a 50 ms yield for other processes.

    It's not what I'm looking for. I may be able to make an interrupt driven USB serial device project. But what I learned today, is that there's no short path for that. I need to learn the low level USB APIs.

  • Oki:

    image

    It registers an exclusive service handler. If I try to and add a shared interrupt handler, I get an assert.

  • in reply to shabaz

    It was a long shot :). 

    It is bound to not work, because the USB data lines aren't connected to UART0. They are connected to the USB data pins of the RP2040. The physical uart0 peripheral is connected to pins 0 and 1.


    The stdio usb wrapper just simulates it is uart0, so that programs using stdio read/write API don't see difference. For lower level use, with interrupts, the stdio mapping doesn't work.

    You can enable both in the makefile though. But the trigger only fires if I simulate traffic on pin 1. Not on the USB port.

  • in reply to shabaz

    I will try it. Meanwhile I've created a test bed, bare metal.

    It's the uart_advanced project from the pico-examples, that I stripped down to just do the receive.
    And the make file adapted to make switching between uart and usb easy.

    Only requires that the environment variable PICO_SDK_PATH is set to the SDK (in my case: C:\Users\jancu\Documents\Pico\pico-sdk), no other dependencies.
    You can set that in the OS, in your startup scripts, or in VSCode, in the User setting CMake:environment

    I'll zip it to the main post