Overview

This page gives an overview of microcontroller form factors typically used by the maker community (at a beginner/intermediate-level). The intention is to assist in finding the right fit for your projects. To achieve this, a high-level introduction of said form factors, and simple feature/property matrices will be presented. Naturally, coverage will be limited by the author's personal knowledge of available options (even if popular in other circles).

The focus is primarily on Adafruit products (tends be available in these popular form factors), but also makes associations to names used by other companies (aliases).

Common form factors (FF)

The following sections present common form factors in decreasing order of size. A few different feature/property matrices will follow (Sorry. Not sure how to link to a lower section).

FF: Mega/Grand Central

Alias: Mega (Arduino), Grand Central (Adafruit)

A slightly larger form factor than the ubiquitous Arduino UNO. The larger size makes it easier to break out more pins available on more capable microcontrollers. The Mega form factor is also used by the Arduino Due, and Giga boards. For the most part, "Mega"-compatible boards should also be able to make use of standard "UNO" shields (stated here).

FF: UNO/Metro

Alias: Uno (Arduino), Metro (Adafruit)

A key development board instrumental in popularizing the entire microcontroller-based DIY/maker movement. The UNO significantly reduced the barrier to entry for microcontroller-based prototyping & development.

Note that there are currently 4 revisions of the UNO format (Currently at UNO R4). Arduino describes the differences over time in this article. Another article describing the differences in more detail is linked here (Not verified by the author, but information looks reasonable).

Other useful information:

• Considered relatively large by Today's standards (though realistically still very tiny at ~2.1x2.7").
• Popularized the idea of stacking "shields" on top of the base microcontroller board.
• ...but newer form factors have become so small, that the trend is to stack add-on boards (like FeatherWings) side-by-side using "doubler", "tripler", and "quadrupler" carrier boards.

FF: Feather

Alias: Feather (Adafruit) / Thing+ (Sparkfun)

Intended to promote cross-compatibility: a standardized pinout should allow people to easily swap-out microcontroller boards, sensor boards, etc. This is basically the same philosophy Arduino uses for the "Wiring" framework to abstract away implementation differences of microcontrollers with respect to code.

The feather form factor is designed with portability in mind. The feather specification requires feather boards to have on-board LiPoly charging (see "VBAT" section).

Of course, the feather form factor also has a nice trade-off between size and functionality (# of pins).

Other useful information:

• Feather history (i.e. why another format?)

FF: Pico

Alias: RPxxxx Pico (Raspberry Pi Ltd.)

"Pico" is the form factor used on the original RP2040 development board. Note that the overall board size is somewhat similar to the Adafruit Feather. Two principal distinctions of the Pico form factor is that is has more pins (2x20) available to break out signals, and does not require built-in LiPoly charging. There are also not many board variants available for those with specific microcontroller preferences (not as much cross-compatibility as the Feather).

Other useful information:

• Adafruit does provide a fairly nice ecosystem of add-on boards for the Pico, named "PiCowbell".
• The official version from Raspberry Pi does not include a STEMMA QT connector (can be inconvenient for some).
• One of the more cost-effective (yet powerful) solutions available from the Adafruit website at the time of writing.

FF: MKR

Alias: MKR (Arduino) / Others?

Almost feather-like in design: 2x14 pins instead of 16+12 for Adafruit feather (both with a 0.8" header pitch). Also meant to support LiPo without needing an extra shield. Currently appears to be a popular form factor amongst  Arduino Portenta family of products.

FF: Micro

Alias: Micro (Arduino)

Developed in conjunction with Adafruit (as described here). Includes the same ICSP header found on the UNO and Mega boards.

Does not appear to have many variants. Not sure if this format is still actively popular at the time of writing.

FF: Metro Mini

Alias: Metro Mini (Adafruit)

There does not appear to be many variants of this board either. Likely because other form factors developed larger ecosystems (see "ItsyBitsy" below).

A single Arduino-based development board is available on Adafruit's website at the time of writing:

• Metro Mini 328 V2 (5V/16MHz)

Might just be the part you are looking for.

FF: Teensy

Alias: Teensy (PJRC)

Mostly focused on providing higher-performance development boards at a reasonable price. Overkill for many projects (what isn't these days? 🤯️), but a good option when you need the extra performance.

Can be adapted to the Feather ecosystem with this clever board.

FF: ItsyBitsy

Alias: ItsyBitsy (Adafruit)

A relatively small form factor by Today's standard. Somewhat similar in size to the Sparkfun Pro Micro.

• 2x14 (main) pins vs 2x12 for Pro Micro.
• 0.6" pitch between rows vs 0.7" for Pro Micro.
• Pinout not easily made compatible with Pro Micro.

Note that many ItsyBitsy boards also include 5 extra pins on the "far end" of the board. Surely a "vestigial" perk of its Pro Trinket lineage (where FTDI headers were required to access the serial console).

Overall, the ItsyBitsy (like the Pro Micro) feels like a reasonable trade-off between number of pins/functionality, and total size.

FF: Pro Micro

Alias: Pro Micro (Sparkfun) / Kee Boar (Adafruit)

A popular form factor for building keyboards - likely because its size allows for just the right amount of pins for such an application.

At the time of writing, there is only a RP2040 variant of the Kee Boar - probably because RP2040 is a good fit for building custom keyboards. Note that Adafruit tends to favour the similar ItsyBitsy form factor (previous section) for projects of this approximate size. Its ecosystem is noticeably larger.

FF: Trinkey

Alias: Trinkey (Adafruit)

Basically any "trinket" that looks like a tiny "USB-key" designed to do very few, simple operations. That said: Trinkeys are typically built from relatively capable microcontrollers, and can actually do quite alot if you are clever about it.

Adafruit Trinkey QT2040

One Trinkey of particular interest is the Adafruit Trinkey QT2040 (#5056).  In the QT2040 product video, we learn that this board's compact 1"x0.7" form factor (mounting hole pitch: 0.8" x 0.5") coincides with the majority of Adafruit's STEMMA-QT sensors.

Also, in the HTU21D-F video, Lady Ada mentions "[...] trying to get all our boards into the same form factor, same pin order, same [QT] connector [...]". The fact that sensor boards are getting this "QT glow-up" (aka getting "STEMMA-QT-ified") means that the Trinkey QT2040 is ideal for building up a compact sensor stack.

Another noteworthy part of the QT2040 product video talks about using the QT2040 as a "great USB Trinkey friend for connecting to any I2C device". Here are some related resources:

• U2IF python example for Trinkey QT2040
• U2IF python example for Feather ThinkInk (U2IF not exclusively for Trinkey version)

With a more standardized 1"x0.7" form factor (than most other Trinkeys), the QT2040 might just be the ideal solution for your project. Note that, at the time of writing: no other Trinkey/microcontroller (I think) comes with the somewhat standard mounting holes used in Adafruit's newer sensor devices.

FF: XIAO/QT Py

Alias: XIAO (Seeed Studio) / QT Py (Adafruit)

Let's face it: most projects don't need tons of I/O pins, and having a quarter-sized board do all the heavy lifting is hard to beat.

• Form factor is big enough for hand soldering by most (no need to tool-up & learn SMT techiques).
• Still contains most of the required supporting circuitry like voltage regulation and supply filtering.
• Can be concealed easily inside almost any project box!

Feature matrices

Matrix: Common form factors

The following table compares some of the seemingly more standard/active form factors for microcontroller development boards:

NOTE:

• Kee Boar: pin-compatible with Sparkfun Pro Micro boards.
• ItsyBitsy: not pin compatible with {Kee Boar/Pro Micro} boards.

Matrix: Special form factors

Some applications (ex: education, wearables) found niches with less typical form factors:

Matrix: Deprecated

This section mentions some additional form factors you might encounter, that appear to have fallen out of fashion or have been outright deprecated. Unless you have a particular need for these boards, it should be easier to find physically compatible components using one of the other form factors presented in this note.

NOTE:

• The Pro Mini appears to be active at SparkFun, but retired from the Arduino line.
  • Pro Mini is also still sold at Adafruit store at the time of writing.
• The "Trinket" line seems mostly deprecated. From product pages: "bit-bang USB technique it uses doesn't work as well as it did in 2014, many modern computers won't work well"
  • Note: Trinket M0 still recommended for people needing the Trinket form factor.

Additional resources

• Make: Boards Guide

🎞️ Videos

• How to pick the best microcontroller for your project (Stern, B. / DigiKey)
• Choosing a Board for CircuitPython A Few Important Considerations (Gallaugher, J.)

Learn guides