MetroSnek — snek on Metro M0 Express

When I first mentioned Snek a few months ago, Phillip Torrone from Adafruit pointed me at their Metro M0 board, which uses an Arduino-compatible layout but replaces the ATMega 328P with a SAMD21G18A. This chip is an ARM Cortex M0 part with 256kB of flash and 32kB of RAM. Such space!

Even though there is already a usable MicroPython port for this board, called CircuitPython, I figured it would be fun to get Snek running as well. The CircuitPython build nearly fills the chip, so the Circuit Python boards all include an off-chip flash part for storing applications. With Snek, there will be plenty of space inside the chip itself for source code, so one could build a cheaper/smaller version without the extra part.

UF2 Boot loader

I decided to leave the existing boot loader in place instead of replacing it with the AltOS version. This makes it easy to swap back to CircuitPython without needing any custom AltOS tools.

The Metro M0 Express boot loader is reached by pressing the reset button twice; it's pretty sweet in exposing a virtual storage device with a magic file, CURRENT.UF2, into which you write the ROM image. You write a UF2 formatted file to this name and the firmware extracts the data on the fly and updates the flash in the device. Very slick.

To make this work with AltOS, I had to adjust the start location of the operating system to 0x2000 and leave a bit of space at the end of ROM and RAM clear for the boot loader to use.

Porting AltOS

I already have an embedded operating system that works on Cortex M0 parts, AltOS, which I've been developing for nearly 10 years for use in rocketry and satellite applications. It's also what powers [ChaosKey])(

Getting AltOS running on another Cortex M0 part is a simple matter of getting clocks running and writing drivers.

What I haven't really settled on is whether to leave this code as a part of AltOS, or to pull the necessary bits into the Snek repository and doing a bare-metal implementation.

I've set up the Snek distribution to make integrating it into another operating system simple; that's how the NuttX port works, for instance. It does make the build process more complicated as you have to build and install Snek, then build AltOS for the target device.

SAMD21 Clocks

Every SoC has a different way of configuring and wiring clocks within the system. Most that I've used have a complex clock-tree that you plug various configuration values into to generate clocks for the processor and peripherals.

The SAMD21 is simpler in offering a set of general-purpose clock controllers that can source a variety of clock signals and divide them by an integer. The processor uses clock controller 0; all of the other peripherals can be configured to use any clock controller you like.

The Metro M0 express and Feather M0 express have only a 32.768kHz crystal; they don't have a nice even-MHz crystal connected to the high-speed oscillator. As a result, to generate a '48MHz' clock for the processor and USB controller, I ended up multiplying the 32.768kHz frequency by 1464 using a PLL to generate a 47.972352MHz signal, which is about 0.06% low. Close enough for USB to work.

At first, I typo'd a register value leaving the PLL un-locked. The processor still ran fine, but when I looked at the clock with my oscilloscope, it was very ragged with a mean frequency around 30MHz. It took a few hours to track down the incorrect value, at which point the clock stabilized at about 48MHz.


Next on the agenda was getting a USART to work; nothing terribly complicated there, aside from the clock problem mentioned above which generated a baud rate of around 6000 instead of 9600.

I like getting a USART working because it's usually (always?) easier than USB, plus demonstrates that clocking is working as expected. I can debug serial data with a simple logic analyzer. This time, the logic analyzer is how I discovered the clocking issue -- a bit time of 166µs does not equal 9600 baud.


While I like having USB on-chip in the abstract, the concrete adventure of implementing USB for a new chip is always fraught with peril. In this case, the chip documentation was missing a couple of key details that I had to discover experimentally.

I'm still trying to come up with an abstraction for writing USB drivers for small systems; every one is different enough that I keep using copy&paste instead of building a driver core on top of hardware-specific primitives. In this case, the USB driver is 883 lines of code; the second shortest in AltOS with the ATMega32u4 driver being slightly smaller.


The only hardware that works today is one USARTs and USB. I also go Snek compiled and running. Left to do:

  • Digital GPIO controls. I've got basic GPIO functionality available in the underlying operating system, but it isn't exposed through Snek yet.

  • Analog outputs. This will involve hooking timers to outputs so that we can PWM them.

  • Analog inputs. That requires getting an ADC driver written and then hooking that into Snek.

  • On-board source storage. I think the ATMega model of storing just one set of source code on the device and reading that at boot time is clean and simple, so I want to do the same here. I think it will be simpler to use the on-chip flash instead of the external flash part. That means reserving a specific chunk of that for source code.

  • Figure out whether this code is part of AltOS, or part of Snek.