Carrier boards are important for users of the Raspberry Pi Compute Module 4, because in its natural state it lacks any I/O options beyond onboard wireless, and some boards don’t even have that. If you want USB, or PCIe, or even some storage beyond any on-board flash you may have specified, you need to connect to a carrier board. Raspberry Pi offer its own Carrier Board, which breaks out everything you can think of. It’s unlikely, however, that you’ll use every connector on a board, so a new release from Mirko Electronics on GitHub, the PicoBerry, adds just a USB-C port and GPIO pins to the module, for minimalist projects.
PicoBerry (tiny @Raspberry_Pi CM4 carrier board) - today project published as Open Source Hardware (OSHW) under CERN OHL v1.2 (Open Hardware Licence).Source and manufacturing files:https://t.co/O3f4cYH3dy@oshwassociation (OSHWA) certification in progress. pic.twitter.com/Na9PPg1gWiOctober 24, 2022
You’ll need one of the CM4 variants with some on-board eMMC storage to get any use out of the PicoBerry, as you’re not going to be able to add any storage using the carrier board. The USB-C port is only for power, like the Raspberry Pi 4, so all the I/O for your project is going to go through the GPIO array, which has 40 pins in the standard Raspberry Pi style.
This means you can connect the CM4/PicoBerry combo to things like LCD screens, HATs, audio DAC boards, and whatever else you can think of that uses SPI, DPI, RS232/485, or any of the other interfaces supported on the Pi’s GPIO. Measuring just 20 x 70mm (0.8x2.75in) the board takes up very little space, and might be suitable for fitting in a tight casing, assuming you don’t need extras like USB ports. If you do need these, you could add something like the USB and Ethernet HAT available from The Pi Hut, but now you’re adding two boards where a single more capable one might do. CM4 compatible boards, like BigTreeTech's CB1, may be able to use the board too, but the CB1 comes with its own carrier, which turns it into a fully fledged RP4-a-like, so again why bother?
There are two user-addressable LEDs on-board, one red and the other green, plus activity and power lights, but that’s really all you get. The PicoBerry is being released as open-source hardware, and there are manufacturing and source files available under the CERN-OHL-1.2 license at Mirko Electronics’ GitHub page. The boards aren’t produced for retail sale, so you’ll need to know what you’re doing to create one.