I fished this keyboard out of the proverbial drawer to try the new CircuitPython on it, with some of the new fancy features that have been added.

First of all, there is now a keypad module for scanning the key matrix in C — since it does it in the background using interrupts, it frees the Python code to display the LED animations much more smoothly.  And it will never miss a key stroke.

Second, there is the new fancy way of enabling and disabling USB devices in boot.py, which means I can by default hide the CIRCUITPY drive and the serial console, and only show them if the upper left key was pressed while the keyboard was being connected. This makes it much easier to tweak the layouts and the code.

Finally, I decided to also test the new custom USB HID descriptors — in particular a descriptor for a "bitmap" keyboard, that lets you report any number of pressed keys, so called NKRO (n-key rollover). I will not lie, I'm not smart enough to come up with my own descriptor, so I copied it from the example generously provided by Jeff Epler. The boot.py file looks like this:

import board
import digitalio
import storage
import usb_cdc
import usb_hid


bitmap_keyboard = usb_hid.Device(
    report_descriptor = (
        b'\x05\x01\t\x06\xa1\x01\x85\xffu\x01\x95\x08\x05\x07\x19\xe0)\xe7\x15'
        b'\x00%\x01\x81\x02\x95\x05u\x01\x05\x08\x19\x01)\x05\x91\x02\x95\x01u'
        b'\x03\x91\x03\x95xu\x01\x15\x00%\x01\x05\x07\x19\x00)w\x81\x02\xc0'
    ),
    usage_page = 0x1,
    usage = 0x6,
    in_report_length = 16,
    out_report_length = 1,
    report_id_index = 7,
)

row = digitalio.DigitalInOut(board.MOSI)
col = digitalio.DigitalInOut(board.A6)
col.switch_to_output(value=1)
row.switch_to_input(pull=digitalio.Pull.DOWN)

if not row.value:
    storage.disable_usb_drive()
    usb_cdc.disable()

row.deinit()
col.deinit()

usb_hid.enable((bitmap_keyboard, usb_hid.Device.CONSUMER_CONTROL))

And the relevant piece of the keyboard handling code looks like this:

    def send_nkro_report(self, pressed_keys):
        """Sends the USB HID NKRO keyboard report."""

        report = bytearray(16)
        report_mod_keys = memoryview(report)[0:1]
        report_bitmap = memoryview(report)[1:]
        for code in pressed_keys:
            if code == 0:
                continue
            if code & 0xff00:
                report_mod_keys[0] |= (code & 0xff00) >> 8
            if code & 0x00ff:
                report_bitmap[code >> 3] |= 1 << (code & 0x7)
        self.keyboard_device.send_report(report)

That's it. That's all it takes to make an NKRO keyboard in CircuitPython now.

Further research is required to automatically switch to the traditional HID keyboard when the host doesn't support the bitmap HID device — like, for example, when you go to your BIOS settings. But there are some promising prototypes for this already.