A 2011 screen for my robot: bringing a Galaxy Note back to life with Linux

My robot has no screen. To give it one, I pulled a 2011 Samsung Galaxy Note out of a drawer. It has a 5.3-inch, 800 × 1280 display and a real pen. It was missing a battery, and an operating system from this century.

Where it started

Once the new battery arrived, the phone booted into what I had put on it years ago: Android 4.4.4 from CyanogenMod. I made a full backup first. Then I installed Nura, the phone Linux distribution from the postmarketOS project, built on Alpine Linux, with a recent kernel.

Without a USB cable

The phone’s USB port still charges, but it no longer carries data to the computer. So everything went over Wi-Fi. The old Android accepts a debugging connection over the network. Through it, I wrote a small boot program, U-Boot, in place of the original kernel. The system itself lives on the microSD card, which I wrote from the computer.

One mistake and I would have needed a cable I don’t have. It booted on the first try.

What didn’t work

1. Wi-Fi. The original phone’s driver made the chip fail at startup. The chip’s generic firmware, with the Note’s own settings file, fixed it.
2. The pen. Nothing described it to the kernel. I added its description: which chip, on which bus, which pin signals a touch. It stayed silent: the driver queries the chip the very moment it wakes it up, too early for it. The fix was to keep it awake all the time.
3. Accuracy. The pen sensor overshoots the screen by about 6% on each side. A small tool with four targets, touched one by one with the tip, computes the correction. It took three passes: one stray touch is enough to throw everything off.
4. The power button. A short press shut everything down. It now turns the screen off, and only a long press shuts the phone down.
5. The battery. The chip’s gauge isn’t set up for this battery. Until it is, the percentage is estimated from the voltage, and the display says so with a “~”.

The desktop

For the interface I chose Window Maker, a desktop modelled on NeXTSTEP: tiles, a menu that opens when you touch the screen, and nothing else. Add a terminal, a virtual keyboard that always sits at the bottom, windows that never spill off the screen, and a pen double tap that counts as a double click. The tip is accurate enough that the cursor became useless: it hides as soon as the pen touches the screen.

What’s next

The phone is ready. Its first real job: becoming the robot’s remote control, with a view through its camera.