STM32: Espruino Pico

[hierophect]

This PR adds support for the Espruino Pico board (Adafruit Link). This board normally runs a javascript interpreter and is updated via a bootloader over a web interface - because Circuitpython depends more on the internal filesystem, this bootloader is not supported and you MUST use the built in F4 ROM bootloader included on all ST chips. This process is documented in the boards/espruino_pico/README.md file.

[hierophect]

@tannewt need PID for this when you have a sec.

[ladyada]

Espruino Pico     VID 0x239A    PID 0x008D  # bootloader
                                PID 0x808D  # arduino
                                PID 0x808E  # circuitpython

[tannewt]

How much of the flash does the Espruino bootloader take up?

[hierophect]

@tannewt the bootloader always takes at least 16KB (one sector) because it can't overwrite itself. If you put the ISR flash after that it leaves only 32KB for the whole filesystem (and it might take even more in the Espruino case, it was kind of hard to tell from their code). It also isn't super well documented how to upload your own binaries with it, since it's designed for use with their online IDE. I don't think it'd be any easier for new users to use it rather than the DFU option, as compared to a UF2 bootloader.

[tannewt]

Ok, it makes to do since the ISRs and bootloader take that much up. I was thinking Espruino owners probably know how to use their Web IDE so it'd be easiest for them. If you still want to do DFU instead, then feel free to merge.

[hierophect]

I'm not an Espruino pro so I could be wrong, but if any come along and think there's a way to get around this, then it'd be easily revisited.

[ladyada]

@hierophect and/or @makermelissa - add these to circuitpy.org next please :)

[makermelissa]

Done. PR is adafruit/circuitpython-org#404

[ladyada]

did a lil hardware test - just checks the button and LEDs

import time
import board
from digitalio import DigitalInOut, Direction, Pull

led1 = DigitalInOut(board.LED1)
led1.direction = Direction.OUTPUT
led2 = DigitalInOut(board.LED2)
led2.direction = Direction.OUTPUT
switch = DigitalInOut(board.BTN)
switch.direction = Direction.INPUT
switch.pull = Pull.DOWN

while True:
    if switch.value:
        led1.value = False
        led2.value = True
    else:
        led1.value = True
        led2.value = False

    time.sleep(0.01)  # debounce delay