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RPi CM4 and Radxa CM3 versions of the board

A project log for Put a Raspberry Pi CM4 into an original iPad

Open up a 2010 iPad, remove its logic board, and fabricate a new board based around the Raspberry Pi CM4.

EvanEvan 01/26/2022 at 07:532 Comments

Since the last time I had a board made, I've discovered several revisions that I needed to make:

- The 5V boost of the bq25895 cuts out long enough to shut off the Pi when you connect USB power.

- I needed extra lanes of DSI going from the CM4 to the SN65DSI83 chip in order for all the clock rates to fall within the ranges allowed by the SN65 and the iPad's display.

- The footprints for my touchscreen FFCs were backwards

I took this opportunity to design a version of the board where I actually used the CM4 instead of having separate 40-pin connector / FFC for DSI / USB port on the board. I threw an M.2 slot on there for NVMe support, as well.

The CM4-based version of my board.

When I was nearly finished with that design, I learned about the Radxa CM3, which is the same form factor as the Raspberry Pi CM4 but has a third board-to-board connector providing several features that would be quite convenient for this application:

In addition, the Rockchip RK3566 that the Radxa CM3 is built around actually seems to have the ability to sleep, which is especially important on a tablet. On the CM4, I'll have to suspend to disk or just shut down or something.

I think if I the Radxa CM3 had existed when I started this project, I might've chosen it over the Pi CM4, as it should require a lot less glue circuitry to make it work in this application. (I had originally considered the Pine64 SOPine and the Nvidia Jetson Nano, but I think I excluded these because the SO-DIMM connectors that they require wouldn't fit between the case and the screen.) As a bonus, the RK3566 is less tall than the BCM2711, which may buy me a little vertical room, depending on the other components on the CM3.

I've also designed a version of my board based around the Radxa CM3. Once I figured out the position and orientation of the third connector on the CM3, this wasn't too bad -- it has far fewer components than the CM4 version of the board. It's essentially just connectors, the touchscreen controller, a buck-boost converter for the NVMe slot, and some audio switches.

Radxa CM3-based version of my board

Since the Radxa CM3 / RK3566 clearly has less software support than the Pi (I think as of the time of writing, you can't even get HDMI output, let alone LVDS.), I'm planning to hedge my bets and have both boards made.

Discussions

dtoy wrote 01/31/2022 at 22:20 point

This is an incredibly enjoyable read. In another life I’d happily (attempt to) follow in your footsteps upcycling two iPads (2 and 3) I have lying around, but luckily I also still have use for these using their original firmwares and accessories. I will start keeping my eyes open for any broken model 1s though. Out of curiosity, was a model 2 or higher ruled out for this project just because of availability or were there physical or perhaps technical constraints as well?
Considering the image quality of the iPad 3 retina screen, as well as its younger age (~less mileage on the batteries?), my impression is it would be even more a worthwhile investment for turning it’s innards into a modern day device.  I am not at all confident that there would be any significant demand for such a product, but the thought is intriguing nonetheless how a world would look where one is able (and it is economically viable) to upgrade an iPad’s capabilities to a modern day device 10y after having originally acquired it. Makes me think of what a manufacturer like FairPhone is attempting to achieve, although they take their ethos to quite another level than merely allowing for sustained product upgrades of course.


Keep up the good work!
 

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Evan wrote 01/31/2022 at 23:02 point

I'm glad you're enjoying it!

I started with the iPad 1 because that's what I had on hand (though compared to all the other costs of this project, the iPad itself is pretty cheap). I think you're right that an iPad 3 would lead to a more modern end product. I think it would be a step up in difficulty, though: based on the iFixit teardown[1], the logic board on the iPad 3 looks pretty narrow. If it's narrower than 40mm (or thinner than about 6mm), it would be tough to fit the CM4 inside. Comparatively, the iPad 1 is pretty spacious inside.

To make it work, I think you'd have to build a board around a system-on-chip instead of a system-on-module, so that you could squeeze the parts into the right shape. Potentially doable by a hobbyist[2], but if you need to go beyond 4 layers to make it fit, then each PCB revision gets expensive.

1. https://www.ifixit.com/Teardown/iPad+3+4G+Teardown/8277

2. https://jaycarlson.net/embedded-linux/

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