What is it?

This board is a electrically similar to a Adafruit Feather RP2040, 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.

Using the popular RP2040 microcontroller from the Raspberry Pi Foundation, you get dual ARM Cortex-M0+ cores @ 133 MHz, 264 kB on-chip SRAM, the powerful PIO peripheral and on this board 4 MB of QSPI flash!

The RP2040 comes with built-in UF2 bootloader for easy flashing, and the board has CircuitPython with Wiznet W5500 drivers pre-installed, plus a poe_featherwing.py module that sets the Ethernet connection up for you.

A special feature of this board is a custom chip to enable the familiar single-press to reset, double-press to enter bootloader button! So from a user experience point of view, it behaves exactly the same as the M4-Shim.

The relevant differences versus the Feather RP2040 are omission of the battery support circuitry (which did not seem necessary in a PoE design), no Qwiic / STEMMA QT connector, no SWD footprint and 4 MB of SPI flash. Note also that the board pins are connected differently to the chip so as to fit the tiny board area available. When using CircuitPython, MicroPython or the Arduino Pico core, the pins use a custom mapping so the names map to the same board pins as the Feather RP2040, but when using a different development environment that uses the RP2040 GPIO pin numbers, you will have to adjust them for this board.

Features

• Feather-like physical shape, optimized for use with PoE-FeatherWing
• Powered by the RP2040 dual core 133MHz Cortex-M0+ with 264 KB of RAM
• 4 MB of fast QSPI flash with 100K erase cycles
• UF2 bootloader for easy Arduino, MicroPython and CircuitPython compatibility
• CircuitPython with W5500 driver pre-installed to ease integration with PoE-FeatherWing
• Two USB variants: horizontal or vertical Micro-B USB

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 RP2040-Shim on top, with the USB facing up.

A 3-PCB stack would not work well with the standard USB direction: if the RP2040-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 RP2040-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 RP2040-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. After the microcontroller on the M4-Shim became unobtanium during the component shortages, I needed an alternative, and the RP2040-Shim was born!

What makes it special?

The RP2040 is a very powerful microcontroller. You could use this board in place of a Feather RP2040 in many projects, if you want to have a lighter board, or if the Feather 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.

Pinout

The following diagram shows the pinout of the RP2040-Shim:

The outer labels document the CircuitPython, MicroPython and Arduino pin labels, which are made compatible with the Adafruit Feather RP2040. However, the actual GPIOs used to connect to these pads differ, so for Pico SDK and other development environments that use the GPIO numbers directly, this will have to be taken into account. The grey labels indicate RP2040 chip pin numbers the pads connect to and the IOxx labels the respective GPIO numbers.

What is included?

This comes with the RP2040-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 RP2040-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.