Lennart401 / PortentaTouchScreen

Library for using a 4-wire resistive touch screen with the Arduino Portenta H7

⚠️ The portenta breakout board or any other access to the high density connectors pins is required to successfully connect the touchscreen when also using the SPI1 (default SPI) interface

Arduino Portenta H7 touchscreen library

(c) 2021 Lennart Popkes.

Distributed under the MIT License.

Introduction

This is a very basic library for interfacing a 4-wire resistive touchscreen with the Arduino Portenta H7.

Due to some pin restrictions from the STM32H7 and a few Arduino API bugs, it is currently (Mar 2021) not possible to use a standard touchscreen driver like the Adafruit TouchScreen Library. You can read about the issue at the bottom of the document.

This solution uses the mbed HAL layer and does not suffer from the software problems listed below.

How to use

1. Connect your 4-wire touchscreen to 4 pins of the arduino.
   • The X- and Y+-wires need to be connected to analog I/O pins. The breakout pin A6 however is the only usable non-high-density analog pin on the portenta. For the second analog pin, @hpssjellis used A7 on the high density connectors to get the example working. As I do not have the portenta breakout board as of now, I could not test this so for.
   • The Y- and X+-wires can be connected to any I/O pin
2. Caution! Do not use any of A0-A3 pins, since these are analog input only! (See below)
3. In your code: include PortentaTouchScreen.h
4. Instantiate a PortentaTouchScreen passing all 4 pins to the constructor. The library assumes a default resistance of 360 ohms across the x-Plate, which is around what most touchscreen will probably have. If needed, it can be passed as the 5th parameter.
5. When neccessary:
   • Get the current touch point using getPoint. It contains the x- and y-coordinates and the pressure z. You can use the Arduino-method map to scale the values to your screen size (like in the basicTouchScreen example). You might be able to use readTouchX, readTouchY and pressure, but they can be a little buggy and might return unwanted results.
   • A touch is sensed if the value returned is greater than pressureThreshold. This might have to be adjusted.
   • The values from getPoint have 11 bit accuracy (0 to 2047).

PortentaTouchScreen and LVGL

The touchscreen can be used as an input device for LVGL. Write your own input device callback using the input type of LV_INDEV_TYPE_POINTER.

The issues with the Portenta H7

• Pins A0-A3 are analogue input only (see chapter Pin Descriptions in the STM32H747xi datasheet, pins PA0_C, PA1_C, PC2_C, PC3_C)
• Once using pinMode() on any of the other analogue pins (A4-A6), no analogRead() will be possible anymore or at least return no sensible value
• Pin A5 (PC3) is connected to Pin D8, which is also the default SPI1 MOSI. This means that as long as you have any SPI devices (like an ILI9341 TFT/LCD) connected, pin A5 is basically useless
• Pin A4 (PC2) is connected to Pin D10, which is also the default SPI MISO. This means that as long as you have any SPI devices (like an ILI9341 TFT/LCD) connected, pin A4 is basically useless.
• With Pins A0-A5 being practically useless for this scenario, it is essentially impossible to connect a TFT/LCD with a resistive 4-wire touchscreen to the portenta without some hacking.