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The line sequence of the trackpoint module and the pins of the PTPM754DR chip


This article was written with the assistance of Google Translate ;D
1. Preface
I've always wanted to add mouse functionality to my keyboard, and the IBM/Lenovo Thinkpad pointing stick is one of the options. There are some disassembled trackpoint modules on the Internet, but basically there is no line sequence definition. The main control chip PTPM754DR commonly used in such modules cannot find the data sheet. After consulting the Chinese and English forum articles, I finally know the discrimination method, and by the way, I understand the pin definition of the PTPM754DR chip.
I got help from this forum along the way, so I report the results back to this forum.
Note that this article only provides a method and idea to identify the line sequence. When encountering different pointing stick modules, specific problems need to be analyzed in detail, and the line sequence described in this article cannot be used mechanically.

2. PTPM754DR chip pin definition
The basic principle of trackpoint is to convert the pressure of the finger on the keycap into an analog quantity, which is then collected and converted by the main control chip, and finally transmitted to the computer through the PS2 protocol to realize the cursor movement. The first obstacle to DIY trackpoint modules is that the disassembly module generally does not have a line sequence description of the external interface. If you know the pin definition of the main control chip, you can quickly understand the interface line sequence through the connectivity between the main control chip and the external interface.

Here, the definition of the main function pins of the PTPM754DR chip is released first, which is derived through reverse deduction. The process of deriving and judging is described later.
Pin2——The data pin of the PS2 interface, DATA
Pin5——Power-on reset pin of PS2 interface, RST
Pin8 - Power - GND
Pin26 - Power + VCC
Pin28——The clock signal pin CLK of the PS2 interface
Pin29~31——The mouse button signal of the pointing stick module (presumption). Some pointing stick modules have three buttons on the left, middle and right of the mouse. I don't know if it means that connecting the micro switches to these three pins can act as the three buttons of the mouse.

Be sure to use the reset circuit described below, otherwise it will not work. As long as you do not connect the positive and negative poles of the power supply wrongly, even if the DATA and CLK are reversely connected to each other, there will be no danger, but the module will not work.

3. How to judge the line sequence of the trackpoint external interface
In fact, knowing the pinout of the PTPM754DR chip in Chapter 2 can handle most of the trackpoint modules.
If you want to know, please use a browser with a translation plugin and browse my Chinese blog.
Thanks for this forum for helping me.


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