Microchip SAMD21 XPLAINED Pro does not start up

[SirTobias]

Hi,

I have this Microchip board to run micropython on it. I have build the current master branch with
make BOARD=SAMD21_XPLAINED_PRO

but have the same problem as described in issue 13790.
When I connect the USB to the DEBUG port, the power status LED switches on and I can see the flash drive mounted. When I copy the firmware file (UF2) format, nothing happens until I plug it out and in the USB again with the mounted flash drive and the firmware vanished. When I instead power via TARGET USB, the power status LED also switches on, but nothing happens: Neither a flash drive is coming up after pressing twice the reset button nor a connection to the /dev/ttyACM port is possible.

IN MPLab X IDE I can see the board and tried to flash it with the firmware in hex format via, but the Micropython bootup procedure does not work.

Does anybody have an idea? Does the PA03 jumper has to be altered? It is now shortened to USB_ID.

The revision of the board is A09-2130/04

Thank you very much in advance.

UPDATE: When I connect in Debug mode, the /dev/ttyACM0 port is there, and the yellow status LED switches on when I connect via picocom or screen but nothing shows up in the terminal.

[robert-hh]

It seems that at the debug port you can only load .hex files and maybe .elf files. The debug port is NOT connected to the SAMD21 chip. It is connected to the DEBUG mcu.
I do not own this board, so I cannot verify this hint: I created a .hex file of the firmware, enclosed in the attached .zip file. You could try to load it through the debug port. If it works, REPL will be available at the target port.

firmware.zip

[SirTobias]

Dear Robert, thank you for your answer. I tried your firmware hex file, but unfortunately, it did not work. Again, when I flash it via MPLab IPE and connect/switch after flashing to the TARGET USB, only the power LED is switched on, nothing else.
When I connect or leave it to the DEBUG USB port, I have /dev/ttyACM0 as a serial port, which I can connect to, but the terminal is left blank and does not react to any key input. The only difference is that as long the connection is active via picocom, the yellow status LED is activated.

I would raise this as an issue. As long as this does not work, I would not recommend this board for any usage with micropython.

[robert-hh]

It looks like an appropriate bootloader is missing. At the microchip forum there is some discussion about that, for example https://forum.microchip.com/s/topic/a5C3l000000UhUBEA0/t180270
You need a 8k sized bootloader.
You could also try to install one of the arduino bootloaders. https://github.com/arduino/ArduinoCore-samd/tree/master/bootloaders

P.S.: The SAMD21 XPLAINED Pro is indeed a little bit of overkill for using MicroPython. The Adafruit ItsyBitsy M0 is more convenient and has in addition a separate flash device for the file system.

Edit: This link looks interesting as well: https://forum.arduino.cc/t/burning-bootloader-on-to-samd21-xpro/918415

[robert-hh]

There seems to be a problem with the board or flash chip. After a power cycle all sectors of the flash are write locked. I do not see any activity at the interface to the flash that's causing this. After running the unlock-script below the flash is unlocked and survives hard reboots until a power cycle.
@SirTobias Could you try if you have the same problem with your board?

from machine import SPI, Pin, soft_reset
from time import sleep_ms

def wait(spi, cs):
    buf = bytearray(2)
    for i in range(1000):
        cs(0)
        spi.write_readinto(b"\x05\x00", buf)
        cs(1)
        if (buf[1] & 0x01) == 0:
            break;
        sleep_ms(1)
    return buf[1]

# instantiate SPI and CS objects
spi = SPI(5, mosi="PB22", miso="PB16", sck="PB23")
cs = Pin("PA13", Pin.OUT, value=1)

# get the initial status
status = wait(spi, cs)
print("Status: ", hex(status), "locked" if status & 0x0c else "unlocked")

if status & 0x0c:
# unlock all sectors
    cs(0)
    spi.write(b"\x06")
    cs(1)
    cs(0)
    spi.write(b"\x01\x00")
    cs(1)
    # get the final status
    status = wait(spi, cs)
    print("Status: ", hex(status), "locked" if status & 0x0c else "unlocked")
    soft_reset()

Edit: It turned out as well that the board cannot operate the flash with 24MHz SCK. So it's reduced to 12Mhz.

[robert-hh]

Just another variant of the above generic bootloader with ID's and boot label indicating the xplained pro board. Just text changes.
If you have already MicroPython running you can use the .uf2 variant uploaded though the target port. Then you have to reinstall MicroPython.

bootloader-xplained-pro.zip

[robert-hh]

Finally I moved the unlock code into flash. It seems to be a standard behavior of that chip to have the flash locked after power up. Unlocking does not hurt if it is already unlocked.

firmware_xplained_pro.zip

[robert-hh]

This morning I replicated the steps for installing the bootloader again. I could not get .bin files installed by copying them onto the XPLAINED drive. Only Intel Hex files work. So there is still the step needed to convert the .bin file to an Intel HEX format.

bootloader-xplained-pro-v3.16.0-15-gaa52b22.zip

[SirTobias]

Dear Robert, thank you very much for the extraordinary help. That worked out.
I could also use your first bootloader answer (https://github.com/orgs/micropython/discussions/15886#discussioncomment-10771710) the upload procedure for micropython and this worked out perfectly. Here, I could also use my self-compiled micropython for the samd21 xplained pro board. For the connection, I just have to connect via picocom /dev/ttyACM0 -b 1200. It worked out of the box to get the prompt ready.

[robert-hh]

Thank you for the feedback. I can now make a pull request. Note that you can use higher baudrates in the communication.
The version of the firmware that I posted uses the external flash for the file system and offers more MicroPython features.