The Raspberry Pi Compute Module 4 was a game changer. It had a new form factor, multiple configuration options, and PCIe compatibility. The beauty of the Compute Module 4 is that it is designed to be embedded in a project, but in order to develop a project we need to use a card that disconnects all USB, HDMI, GPIO etc before use.
The official carrier board has the most comprehensive selection of features, but this slightly increases the size. Sourcekit’s $14.50 PiTray Mini brings the Compute Module 4 together into something akin to the traditional Raspberry Pi form factor, but it offers the chance to upgrade our Raspberry Pi as new models arrive. With this tempting option dangling in front of us, we had to put one on the couch for a test.
Hardware Specification:
| GPIO | 40 Pin Raspberry Pi Compatible GPIO |
| Ports | 1 x Gigabit Ethernet |
| 1 x USB 2.0 Type A | |
| 1 x HDMI 2.0 | |
| 1 x USB – C (for power and eMMC flashes) | |
| 1 x MicroSD connector | |
| Buttons / Switches | eMMC boot selector switch |
| Run Reset Switch | |
| Dimensions | 3.3 x 2.2 in (85 x 56 mm) |
Design and use of the PiTray Mini
The design of PiTray Mini is clearly influenced by the typical Raspberry Pi Model B form factor and while there is an overall look along with 40 pin GPIO and M2.5 screw holes to attach a HAT to the board, the agreements expired. On the left we see a single USB-C port, which is used to power the board and flash your Compute Module 4’s eMMC.
Just next to the USB-C port is a micro SD card reader with a push/push mechanism, ready to eject your card halfway across the room. At the bottom left of the board is the eMMC Boot switch, and, as you may have guessed, it’s used to enable booting from the eMMC flash in Compute Module 4 boards.
If you are using a Compute Module 4 Lite, it must be set to Off and a microSD with Raspberry Pi OS must be inserted into the reader. At the far right of the board is a single HDMI 2.0 port, a USB 2.0 port, and Gigabit Ethernet. There are no CSI or DSI connectors for the official cameras or display.
We tested the PiTray Mini with an 8GB Compute Module 4 Lite and a Compute Module 4 with 1GB RAM. First we booted the Compute Module 4 Lite from a micro SD card and everything went well. It was like using a typical Raspberry Pi 4, but with three less USB ports. We then tested a Compute Module 4 with eMMC flash storage by first using the PiTray Mini as a carrier card to flash our module. By following the instructions to mount the eMMC module using the usbboot application, we simply flashed the latest Raspberry Pi OS image directly to the eMMC module. We flipped the eMMC boot switch to On and turned the board on and within moments we saw the familiar Raspberry Pi desktop.
Since PiTray Mini has a 40-pin GPIO, we tested it with a range of boards from Pimoroni and Adafruit. First we tested the Unicorn HAT, a 64 Neopixel grid on a HAT. This installed without any problems, but the sample code didn’t work, claiming that the Compute Module 4 was an unknown board type. The same problem hit us with Adafruit’s MPR121 board. Pimoroni’s Explorer HAT Pro, on the other hand, worked flawlessly. Are these problems caused by the PiTray Mini? No, these compatibility issues are related to Compute Module 4 itself. Basic GPIO control, for example via the Python RPI.GPIO and GPIO Zero libraries worked without any problems.
Possible uses for the PiTray Mini
With a Raspberry Pi layout, you’d think the PiTray Mini would be the ideal means of having the power of a Raspberry Pi 4, with the option to upgrade to later models when they arrive. This is largely true and if you need this flexibility, the PiKit Mini is a strong option at this price.
If you’re looking for an inexpensive way to get a home server, the Gigabit Ethernet and powerful quad-core processor are a plus, but the USB 2.0 port slows down your disk access. The PiTray Mini is a strong contender for an expandable media center.
Bottom Line
We had high hopes that PiTray Mini would be the start of an upgrade route for a Raspberry Pi that would be compatible with future Compute modules. Our hopes were not thwarted as PiTray Mini simply works with our Compute Module boards. But if we were hoping to use the GPIO with the best Raspberry Pi HATs, at this point there are some issues that PiTray Mini has exposed, namely the compute Module 4 card compatibility is patchy at best. For the price, PiTray Mini is worth investigating, but we wouldn’t base a project on it just yet.