ZMK-ThinkPad - Part 3: Connector arrived, and then there was light
Summary
Backlight on Revision A does not work. The power circuit browns out. So I turned to my trusted friend: National Semiconductor Analog Discovery 2, plugged in the T480 keyboard, and backlight was lit. It wasn’t pretty, but at least it works.
12 pins into a 14-pin connector
The 14-pin connector is a generic FFC connector.
As much as I hoped that it’d just fit like the 40-pin FFC connector, the dimension don’t quite work. There needs to be some rejigging and alignment. I 3D printed a guide.
Guide STL: U-shaped guide
The guide centres the FFC cable within the 14-pin connector. Just need to eyeball the track alignment to the pins afterwards.
Driving the backlight
With the cable sitting still, the Analog Discovery 2 drove the T470 backlight.
While the backlight powers off +5V, its backlight intensity is controlled by PWM.
| Duty cycle | Power usage |
|---|---|
| 80% | 595mA |
| 85% | 620mA |
| 90% | Brown out |
The new Rev B PCB will need to test out different current PWM ranges to suit both USB 2.0 and USB 3.0 with USB-C connector.