I am a huge fan of the SAMD21 boards that Adafruit is producing. What a huge step up from the ATMEGA328 or 2560. 48MHz, 32 bit ARM Cortex-M0+, and loads of flash and ram.

I have several of these boards and am dabbling with CircuitPython (I finally a compelling reason to learn Python!). I have a couple/few each of Circuit Playground Express, Feather M0 Express, Gemma M0, and the most recent: the Trinket M0.

Some advantages of the Gemma and Trinket boards is their small size. One effect of this is that there is limited board space, which means that neither of these boards include an exteral SPI flash chip that the “Express” boards include. This means that you are limited in the amount of application code (in Python) the board can contain. Note that if you’re working in C/C++ you will be able to fit far more application code; most likely you will have more than enough space for your code. Now, since the Trinket and Gemma boards are also limited in their I/O connections, having a limited amount of code is probably not going to be a huge problem.

Even so, my first thought when I ordered some Trinket-M0s was to wonder what would be involved in adding an SPI flash chip. After some research, hacking, and digging through code I have a working Trinket M0 Express.


I used the 2 MByte S25FL116K0XMFI043 SPI Flash chip which I picked up at Digikey. This is the same chip that is on the Feather M0 Express boards I have.

Getting the SPI flash chip connected to the SAMD21E18 was straightforward. By comparing the schematics and board specific code of the Feather M0 Express and the Trinket M0, it was clear that I just had to connect the flash chip to a set of unused SERCOM capable pins. I chose pins 17-20 (PA16-PA19). Additionally I used pin 14 (PA11) for the SPI select line. Connection is simple, with direct connections between the MCU pins and the corresponding SPI flash pins.

While figuring out what connections were required was easy enough, making them was a bit more challenging. I’m very handy with a soldering iron, including SMT work, but this was touchy. The MCU has very little pin surface exposed, and they are very close together. Using wire-wrap wire and an SMT iron tip it wasn’t too hard, but was still moderately daunting.

For my initial prototype I mounted the flash chip on an SOIC-8 breakout (from Adafruit) so that I could plug it into a breadboard to make hookups easy. I likewise added headers to the Trinket and mounted it to the bread board as well. This let me easily tap into the ground and 3v output from the Trinket, as well as hold it solidly in place relative to the Flash chip. This minimized any forces on the solder joints on the MCU pins.

trinket and flash mounted

To make the connections, I first put the Trinket in a board holder and soldered short lengths of wire-wrap wire to the MCU pins I was using.

wires on MCU

With that done, it was a simple matter to solder the other ends of the wires to the flash chip. I added short strips of header to the breadboard to make this even simpler. Note that I added an external reset pushbutton to minimize the chance of jostling the wires and their delicate connection to the MCU as much as possible.

connected and working


With the hardware work done, it was time to add a definition of the “Trinket M0 Express” board to circuit python. I forked Adafruit’s repo and copied the tinket_m0 directory to trinket_m0_express and made tweaks to the mpconfigboard.h and files to reflect the addition of the external SPI flash. I also tweaked conf_access.h and conf_usb.h to change the device name to include the “Express” suffix. Here are the diffs:

>:diff boards/trinket_m0/conf_access.h boards/trinket_m0_express/conf_access.h
< #define LUN_0_NAME "\"MicroPython VFS[0]\""
>#define LUN_0_NAME "\"CircuitPython VFS[0]\""
>:diff boards/trinket_m0/conf_usb.h boards/trinket_m0_express/conf_usb.h
< # define USB_DEVICE_PRODUCT_NAME "Trinket M0"
> # define USB_DEVICE_PRODUCT_NAME "Trinket M0 Express"
>:diff boards/trinket_m0/mpconfigboard.h boards/trinket_m0_express/mpconfigboard.h
< #define MICROPY_HW_BOARD_NAME "Adafruit Trinket M0"
> #define MICROPY_HW_BOARD_NAME "Adafruit Trinket M0 Express"
< #define MICROPY_PORT_A (PORT_PA00 | PORT_PA01 | PORT_PA24 | PORT_PA25)
> // Salae reads 12mhz which is the limit even though we set it to the safer 8mhz.
> #define SPI_FLASH_BAUDRATE (8000000)
> #define SPI_FLASH_PAD2_PINMUX PINMUX_UNUSED // Use default pinmux for the chip select since we manage it ourselves.
> #define SPI_FLASH_CS PIN_PA11
> #define MICROPY_PORT_A (PORT_PA00 | PORT_PA01 | PORT_PA11 | PORT_PA16 | PORT_PA17 | PORT_PA18 | PORT_PA19 | PORT_PA24 | PORT_PA25)
< #include "internal_flash.h"
> #include "spi_flash.h"
> #define BOARD_FLASH_SIZE (0x00040000 - 0x2000)
> #include "flash_S25FL216K.h"
< #define BOARD_FLASH_SIZE (0x00040000 - 0x2000 - 0x010000)
>:diff boards/trinket_m0/ boards/trinket_m0_express/
< LD_FILE = boards/samd21x18-bootloader.ld
> LD_FILE = boards/samd21x18-bootloader-external-flash-crystalless.ld
< FLASH_IMPL = internal_flash.c
> FLASH_IMPL = spi_flash.c

This seems straight forward, but actually involved digging through the CircuitPython and SAMD21 driver code and documentation to figure out how everything fit together. It’s accurate to say that I have a much better understanding of the inner workings of the SAMD sercom functionality and configuration now.

Version 2

Once I had it cobbled together on a breadboard, and the software squared away, it was time to make a more useful version. As before I started by soldering short lengths of wire-wrap wire onto the SAMD21 pins.


This time I wanted a self-contained version, so I piggybacked the flash ship onto the MCU with some epoxy. I also put epoxy over the pins that had the wires soldered onto them to protect the connections. One change I made from this photo before the epoxy set was to rotate the flash chip 180 degrees.


This is the final version with the wiring finished, and the SPI select pullup resister added.



So there it is. With a couple days (total) of research, code spelunking, soldering, and coding I now have an “express” version of the Trinket M0. The final version of the hardware build took maybe a half an hour not counting the time for the epoxy to set. Now I have a tiny M0+ board with 2Mbytes of flash. Now to find something to use it for.

I’m now wondering how hard it would be to redesign the board to put the flash chip on the underside. Since the board design files are open source, I may just give this a try.

Tidying up

I’ve made another Trinket M0 Express, this time focusing on making the wiring a bit tidier:


CircuitPython 2.0 update

It took all of 5 minutes to add my “Trinket M0 Express” support to CircuitPython 2.0 that was just released. All that was required was to add on line for NVRAM support.

9 thoughts on “Trinket M0 Express Hack

    1. Thanks.

      I don’t… it was so trivial. Have a look at the feather M0 Express schematic, connecting the flash chip to the MCU pins as noted in the code diff. Just don’t forget the pullup on the select pin.

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