Raspberry Pi

Microchip AR1100 Resistive Touch Screen to USB

I’m currently working on a project similar to what Rune Kyndal did, which is putting a Raspberry Pi inside an old HP 95LX case. You can see Rune’s project on HACKADAY.IO.

I sourced a 800×320 screen from, that is a good match for the original dimensions of the HP 95LX display. Arguably, the screen that Rune sourced is better at 800×480, but I wanted a display that would allow for the original faceplate to be used so it looks as close to the original as possible. I still need to cram everything into the housing.

One thing that I wanted to be able to do is use the device without a mouse if I want to. That requires getting the touch screen to work. Today I was finally able to get the touch screen in a usable state with the help of tool created by tom-2015 and available on GitHub. The application runs on Linux, which I needed as the screen is at an odd resolution so the Microchip calibration tool will not work for the display. I was able to get Windows to use the display, when connected through HDMI, but was unable to get the correct resolution for calibration.

The rpi-AR1100 application provided on GitHub does the trick but I did have an issue that I needed to work through first. It took a few days of looking through the Microchip datasheet and walking through the code to determine that it was a timeout issue with reading/recognizing the calibration points being touched.

Issue Prior to Calibration

Before Calibrating the screen, the touch did not track to the edges of the screen. It was fine in the middle of the screen but as you got out to the edges, the pointer was not tracking and would not reach to the edges of the screen. Below is an image with a red rectangle showing the approximate bounds of the touch area prior to calibration.

Rectangle showing perimeter of the pointer boundary prior to calibration

The AR1100 works well once it is calibrated. If you calibrate it on the Raspberry Pi using Tom’s rpi-AR1100 utility on GitHub and find that the calibration is not responding to touches, you may need to edit line 185 in AR1100.cpp by changing the timeout parameter passed to the read_data function. I made the following edit to get it working on the Raspberry Pi Zero W.

Changed AR1100.cpp line 185
FROM: int result = read_data((unsigned char *) & response, sizeof(response), & bytes_read, 5);
TO: int result = read_data((unsigned char *) & response, sizeof(response), & bytes_read, 50);

UPDATE: Tom has made this change to the code in his repository. If you find that the application is not responding when touching the screen, you may need to increase this value.

Hope this helps you if you are working with the Microchip AR1100.
BTW: I’m using the Adafruit Resistive Touch Screen to USB Mouse Controller – AR1100, product number 1580.

By richteel

Software and hardware developer who likes learning new things with a passion of sharing knowledge with others.

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