This eReader has a Touchscreen and Backlight!

A project log for FLOSS Book Serving System

An open-source ereader that can display books from an open, self-hosted server

guyrandy-jean-gillesGuyrandy Jean-Gilles 12/15/2023 at 00:050 Comments

The new CM4 and HAT PCBs for the GDEY042T81-FT02 are in and by and large they work. I still have to troubleshoot some hardware/firmware, but the project now has portable power, a touchscreen, and backlight.

Portable Power

First, the 5V output being disabled is resolved in this version of the carrier board. You'll note in the above picture that there is no USB micro populated on the carrier board because of an error during manufacturing. I actually haven't tested charging on this specific version of the carrier board, but it's worked on previous boards so I'm not worried about it.

While still functional, the battery connector, S2B-PH-SM4-TB(LF)(SN), is really tight. So tight, that I have to use pliers to remove the battery once it's connected. I've only disconnected the battery a couple times but it's connector is already starting to deform. The dedicated lipo charger and boost converter I made used a through hole connector, B2B-PH-SM4-TB(LF)(SN), that doesn't mate as tightly. I'll switch out the part in the next revision.

I also realized a power switch, even if it isn't hardwired into the power rails, would be useful. Another change for the next next hardware revision.


While the backlight works, for some reason I cannot get it to reliably dim with the new hardware. When connected to a DC power supply, the backlight dims as expected. But with the new PCBs, I can turn it off and turn it on, but I can't seem to consistently drive it at, say, 50% brightness. I haven't figured out why yet, but I did notice that the TPS61165 driver is drawing a TON of amperage. Around 300 mA worth and I believe it's causing a voltage drop on the HAT. On the carrier board, the TPS63701 boost converter is good up to 1.2A so there's nothing to worry about there.

Even when I power the new hardware with a 10A DC power supply, I'm still seeing unreliable dimming behavior. To rule out a software issue, I bought a development board for the TPS61165. There are also some forums topics that have promising discussion. I'll probably write up a different project log once I resolve the issue.

Also, when I insert a battery pack, the backlight turns on very briefly and then turns off. I think what's happening is the Raspberry Pi GPIO that connects to the control pin of the TPS61165 briefly is pulled high before becoming tri-stated. I plan on adding a weak pull down resistor to the TPS61165 control pin to prevent the blimp during battery insertion.

Ribbon Cables

You'll notice some contortion on the 24-pin ribbon cable in the picture above.I preemptively moved the 24-pin FFC connector closer to the edge of the board based on GDEY042T81-FT02's fitment on existing PCBs I had in hand. I'll probably have to move it back two millimeters or so to make things fit properly.

Next Steps

My next goal is to add a virtual keyboard to the project. I've already prototyped basic typing, but the full implementation will need partial refresh as each key press takes 1.5s. Also, I'm only using black and white for the GDEY042T81, but 4 color grayscale will allow pop overs on key presses and better contrast modals too.

There's also a new HAT+ form factor for the Raspberry Pi 5. I don't have one so I don't think I'll make a new board for it. I'm fairly content with the hardware right now, so most of the future updates will probably be firmware related.

For the latest hardware and firmware updates, please see the projects repository.