TRMNL on a Kindle

I’ve written before about TRMNL - it’s a great project. During a recent cleanout, i found an old Kindle Voyage in the back of a drawer. As well as supporting their own devices, TRMNL offers a “BYOD” licence and, if you manage to get it up and running on a device that nobody else has published before, they even give you back some of the licence fee!

goals

• Put a TRMNL in the hall to show the Tube, bus and train status in the morning before we talk out of the door
• reuse the Kindle to make a low-power TRMNL device
• implement some sort of OTA updates, so i dont have to move it around to update it

jailbreaking the kindle

This was much easier than i thought it would be. The KindleModding site has an easy walk through - basically, you:

1. plug the kindle in to the laptop and fill it with dummy data so that automatic updates dont get downloaded.
2. find the serial number, enter it in to the wizard which will help you pick the correct jailbreak
3. follow the instructions it was really easy. Took a little while, and i wasnt really sure it had finished until a new “Jailbroken” book appeared on the Kindle home screen.

Attempt 1 - KOReader + TRMNL-KOReader

KOReader is a multi-platform ebook application based around “plugins”, and TRMNL provides the TRMNL-KOReader plugin for it. TRMNL recommends this approach as they say it’s “easier”. I installed KOReader, and installed and it worked. But, there were a couple of annoyances - if you accidentally tap on the screen, TRMNL closes (which isnt great in a hallway or wherever), so I raised a PR to allow users to chose the exit mode (single tap, long press). I also raised a PR to enable taps to fetch or advance the current dashboard.

But then i started to hit some more complex problems around wifi resilience and battery life. I spent a little time seeing if i could fix it, but it’s impossible to properly update the wifi lifecycle without changing KOReader and i didnt want that (it’s much more complex than i am comfortable working on). So after pushing my PRs, i abandoned this and moved to KUAL.

Attempt 2 (which worked) - KUAL + TRMNL-Kindle

KUAL (Kindle Unified Application Launcher) is a launcher and menu system for running 3rd party/custom applications and scripts on Kindles. To prove it out, i installed TRMNL-Kindle, and this was pretty slick. I noticed that wifi recovery was a bit flakey - if it failed, it would end up permanently disabled - so i submitted a PR (which has since been merged) to handle this gracefully.

Support remote updates

There’s no over the air update mechanism for KUAL or TRMNL-Kindle. TRMNL-Kindle uses the hardware wake timer to suspend the device between refreshes - this is very energy efficient, but it means that the Kindle is unresponsive, so you can’t just tap the screen to quit TRMNL, and power cycling takes around 5 minutes. So i submitted another PR to detect when the device has been woken significantly before the expected hardware wake time. If it has, it presents a “tap to quit” button for 10 seconds (in case it was woken for another reason), and if the user taps (or presses a hardware button on a non-touch Kindle), it quits TRMNL and cleans up its timers etc.

With this done, i installed Kindle USBNetwork which provides a Dropbear SSH server. After hardening to only accept public key authentication, i can now perform updates over the network without having to plug the Kindle in to my laptop.

Conclusion

Well, i achieved my goals. I jailbroke Kindle Voyage, installed KUAL, TRMNL-Kindle, Kindle USBNetwork with Dropbear, and locked it down with SSH Key authentication. I gave back to the community with a few PRs closing the gaps i found. KOReader wasnt particually bad or broken - it just didnt do what i wanted. TRMNL-Kindle is working just fine.