What is it?

This board is a electrically a clone of the Adafruit Feather M4 Express, but physically optimized to fit on top of the PoE-FeatherWing, filling the empty space around the RJ45 and flyback transformer and allowing the creation of extremely compact Power over Ethernet systems.

Just like the original Feather M4 Express, it is powered by the ATSAMD51J19 - with its 120MHz Cortex M4 with floating point support, 512KB of flash and 192KB of RAM. It also has 2MB of SPI flash on board, which contains the CircuitPython file system by default. The only thing missing compared to the original Feather is the battery charger, which was deemed expendable for Power over Ethernet systems. The board comes with a UF2 bootloader for easy Arduino and CircuitPython compatibility, and has CircuitPython with Wiznet W5500 drivers pre-installed, plus a quick example module that sets the Ethernet connection up for you when used with the PoE-FeatherWing!

What's with the two USB connector options?

There is one option with horizontal USB connector, it has the micro USB in the same place as a normal Feather would have it: facing the opposite direction from the Ethernet jack when mounted on a PoE-FeatherWing.

The other option has a vertical USB connector, facing up from the board. It is intended to be used in systems that stack 3 boards: PoE-FeatherWing on the bottom, an application specific board in the middle (which can have its own connectors facing the opposite direction from the Ethernet jack) and the M4-Shim on top, with the USB facing up.

A 3-PCB stack would not work well with the standard USB direction: if the M4-Shim's USB faces the same way as the connectors on the application specific board, they would likely interfere with each other. Worse, if the middle board is longer with more circuitry extending out from the back, the M4-Shim's USB would become completely unreachable. By facing the USB up, it continues to be easily accessible from the top, no matter what the application board in the middle looks like. The whole M4-Shim "user interface" (button, USB, LEDs) is also nicely clustered together. The USB height is about even with the top of the Ethernet jack, simplifying case design.

Why did you make it?

Because makers are never satisfied. (That tends to be why they make! :))

No sooner had I integrated PoE and Ethernet on a tiny FeatherWing, and someone expressed disappointment that they had to use it in combination with a Feather to provide the microcontroller! They wanted PoE + Ethernet + microcontroller all in one unit.

To be honest, I could understand the desire myself. I love cramming tons of functionality into a tiny footprint. The Feather is a tiny form factor, but once you start stacking various boards together to make something functional, the total size quickly stops being tiny. One of the things that bug me about the PoE-FeatherWing is that there is the giant Ethernet jack, and the giant flyback transformer, with so much unused empty space in between. So I wondered whether I could do something useful with that empty space. Thus the concept of the M4-Shim was born: fill those gaps with circuitry!

What makes it special?

It's a very powerful microcontroller, just like the Feather M4 it mimicks. You could use it in place of a Feather M4 Express in many projects, if you want to have a lighter board, or the Feather M4 is out of stock.

But of course, it really shines when combined with the PoE-FeatherWing, where it literally becomes a zero-footprint solution, adding a lot of processing power without taking up extra volume.

What is included?

This comes with the M4-Shim, two single row 20-pin headers with 6.7 mm pins on both sides that can be snapped to size (to 12 and 16 pin) for installation, and a plastic spacer to maintain distance between the PoE-FeatherWing and the next board stacked on top (either M4-Shim or application specific).

A PoE-FeatherWing is NOT included but can be ordered separately.

Please refer to the pictures to see how the spacer can be installed and what pin arrangements can be used depending on your application. The included pins are good for final assembly of deployed units that are all soldered together, for prototyping and development you might want to use stacking headers instead.