!NEC MPi400: Reverse Engineering the Keyboard Matrix

Overview

To be able to use the handheld, it's important to have a fully working keyboard. The most needed information for this is a complete documentation of the keyboard matrix.

Preparation

To be able to measure the keyboard matrix without destroying the FPCs, I designed a small PCB and ordered it at JLCPCB. Only drawback was the written naming. I initially planned on naming the system !NEC MP900 to show of the old name - after ordering I rethought this and named it !NEC MPi400 in reminisence of the Raspberry Pi 400 computer.

The PCB arrived in pretty good condition with everything assembled. I did this to remove the effort of soldering out of the equation.

The PCB is planned to be a "keeper of notes" so I can directly write on the PCB. So I only have to transfer my notes to a digital copy.

Doing measurements

I've directly marked the rows (left → right) and columns (top → bottom) which are either shorted together or not connected at all.

In the following picture you can see how I did the measurements of the keyboard.

Result

After finishing the measurements, I was left with all keys pressed a thousand times, bent test loops on the PCB, but I have found all keys!

The result of the measurement has been copied over into a simple spreadsheet (QMK Keycodes), which will be used for creating a QMK keyboard file.