I came across Pi-Supply’s Pi-Crust add-on board quite by chance, but it immediately caught my eye as quite possibly the most useful add-on for the Raspberry Pi there is. Having ordered one, received it and soldered all the parts together, I’m no longer sure that it is.
The notion behind the device is very sound indeed. It’s an internal breakout board that doesn’t merely replicate the Pi’s GPIO ports, it re-organises and labels them logically and clearly. It’s also intended to fit snuggly within the form-factor of the Pi itself. This is, frankly, a brilliant idea.
As you can see from the pictures, the assembled Pi-Crust is a small L-shaped board which clips onto the GPIO pins and routes them to a set of well-spaced connectors clearly marked with their function: SPI, UART, I²C, GPIO and so on. This makes accessing the bus you want a doddle and saves you from either looking at a pin-out reference sheet every time you want to connect the Pi to something, or learning the whole GPIO array by heart.
No longer. Now I can simply drop my wires into clearly marked GND, 3V3, SDA and SCL and start sending bits and bytes back and forth between the Pi, a four-digit, seven-segment LED and a light-level sensor. Or SPI’s MISO, MOSI, SCLK et al. Or UART’s RX and TX.
Another particular advantage the Pi-Crust offers is that it uses female connectors rather than male ones, just like the Arduino boards do. This means you can work with simple jumper wires – you don’t need gender-bending intermediary cables – and you’re not going to make contact with the adjacent pin by mistake.
The Pi-Crust doesn’t use the usual 25-pin female IDC connector on its base to grasp the Pi’s pins. Instead it uses an ingenious connector that allows it to sit just a few millimetres above the Pi’s own board. There’s a hole cut in the Crust to accommodate the Pi’s camera connector. The Crust’s own connector is ingenious because without it the female headers on the upper side of the board would reach well above the height of Pi’s own components, ensuring the device wouldn’t fit inside a Pi case.
And that’s one of the Pi-Crust’s raisons d’etre: it’s supposed to lie sufficiently low to fit inside a case. Ergo, you can leave it in place. This appeals to me because, favouring neatness, I don’t like leaving my Pi lying around with an extender cable hanging off it. With the Pi-Crust, I reasoned, I could lose the cable, and just pop the top off my case and start plugging wires into those female headers whenever I needed to.
What sold me was ModMyPi’s claim: “If you have a case for your Raspberry Pi, this board should fit inside it.” This echoes a claim on the Github page of Joe Walnes, the guy who invented Pi-Crust. I’m currently using one of ModMyPi’s cases, selected because it has a readily removable lid, perfect for easy occasional GPIO access. Surely the Pi-Crust will fit into one of the seller’s own cases?
Like heck it will. An not many others either.
Here’s the problem: the Pi-Crust extends a millimetre or two past the edge of the Pi’s own circuit board. Because the Pi fits very snuggly in the ModMyPi case, for instance, this overlap leaves no room for the case’s upper half to close. The internal spars in Pimoroni’s Pibow case likewise get in the way of the Pi-Crust.
About the only good case I know of into which the Pi-Crust will fit without too much bother is Cyntech’s. But putting it in one of these defeats the object of having the Pi-Crust in the first place: the Cyntech case is designed to be held together with four metal screws. Good though it is, it’s not a case for Pi users who want frequent, ready access to the board.
You can sand down the edge of the Pi-Crust, but this won’t get you very far. You can’t wear it away sufficiently for it to allow the ModMyPi case, for one, to close. If you did, you’d inevitably remove half of the pins by which the board’s novel GPIO connector is soldered to the board – not to mention the lines running from those pins to the female headers.
The Pi-Crust is a good idea – a really very good idea indeed. It makes the Pi’s GPIO pins as accessible as they ought to be, by separating out pins by function and by labelling them clearly. At around £13 it’s a little expensive for what is just a circuit board, a PI attachment and 26-odd female connectors, all of which you have to solder together yourself, but it’s worth it for how well it presents the Pi’s GPIO.
The flaw for me is that it doesn’t fit into any of my Pi cases, so it’s not something I can leave sitting on the Pi, which is why I got it in the first place. I’ll certainly make use of it, but because it can’t be a permanent Pi attachment, its utility is unfortunately limited.