Electronic – Interfacing a four wire resistive touch screen with STM32F429DISCOVERY

cmicrocontrollerstm32touchscreen

I'm trying to interface a 4-wire resistive touchscreen to a STM32F429 Discovery board but I can't figure out how it works. I want the MCU to detect a touch so I configured the 4 pins like this:

X+ --> input w/pullup connected to EXTI external interrupt 
X- --> input floating (tristated)
Y+ --> input floating (tristated)
Y- --> output to GND

in this configuration I should be able to detect a touch and trigger an interrupt on the EXTI line so I can enter a ISR and sample the x and y values with the ADC. But when I connect the X- pin the X+ pin goes to near GND. It seems like current flows between X+ and X- (so the X- is not tristated?).
Here's the code I use to configure the GPIO pins:

GPIO_InitTypeDef gpio;
gpio.Pin = GPIO_PIN_2 | GPIO_PIN_1;
gpio.Mode = GPIO_MODE_INPUT;
gpio.Pull = GPIO_NOPULL;
HAL_GPIO_Init(GPIOA, &gpio);

gpio.Pin = GPIO_PIN_0;
gpio.Mode = GPIO_MODE_INPUT;
gpio.Pull = GPIO_PULLUP;
HAL_GPIO_Init(GPIOA, &gpio);

gpio.Pin = GPIO_PIN_5;
gpio.Mode = GPIO_MODE_OUTPUT_PP;
HAL_GPIO_Init(GPIOA, &gpio);

HAL_GPIO_WritePin(GPIOA, GPIO_PIN_5, GPIO_PIN_RESET);

/* initialization using registers, works the same as above */

//  /* Xr and Yu open (input floating) */
//  GPIOA->MODER &= ~(GPIO_MODER_MODE2 | GPIO_MODER_MODE1);
//  GPIOA->PUPDR &= ~(GPIO_PUPDR_PUPD2 | GPIO_PUPDR_PUPD1);
//
//  /* Xl input with pull-up */
//  GPIOA->MODER &= ~GPIO_MODER_MODE0;
//  GPIOA->PUPDR |= GPIO_PUPDR_PUPD0_0;
//
//  /* Yd to GND (output set to 0) */
//  GPIOA->MODER |= GPIO_MODER_MODE5_0;
//  GPIOA->ODR &= ~GPIO_ODR_OD5;

I can't understand what I am doing wrong or how to configure the tristated pins correctly so they don't pull down the X+ pin.

Best Answer

On the STM32F429 Discovery board, PA1 and PA2 are directly connected to the interrupt outputs of the L3GD20 motion sensor chip, which are by default active high. That means, they should output a logic low signal until some (programmable) event occurs. Which, as the part is presumably not initialised, never happens.

The X+ and X- pins of a resistive touchscreen are connected with a resistor, typically with a value below 1kΩ. Connecting X+ to PA0 and X- to a low-level output effectively created a 1kΩ pulldown on PA0.

PA0 is connected to ground through a 220k pulldown resistor (part of the debouncing circuit of the blue pushbutton) on the discovery board as well, so it's not a great choice for an analog connection either.

You can free up PA0 by cutting a solder bridge somewhere, but PA1 and PA2 are apparently directly wired to the motion sensor.

You should consult the User Manual of your board and find 4 unused pins, two of them must be usable as ADC channels. Unfortunately the onboard LCD takes up most of the ADC pins, but there are a few left, namely PA5, PC3, and PF6. Pick two of them for the analog-digital conversion, and find two unused pins for the other two terminals. Opening the predefined STM32F429-Discovery project in STM32CubeMX is perhaps the simplest way to find the usable pins.

Related Solutions

Electronic – Resistive Touch Screens

I think the reading is taken from the other axis. So if you want to read the X axis you apply power/ground to the X wires and read the ADC connected to Y.
If you notice in the code snippet it says power is applied to X axis but reading is taken from CH0 (RA0) and named result_y (I think the diagram may have RC0 and RC3 connections mixed up though):

//Set PORTA To Inputs/High Impedance
TRISAbits.TRISA0 = 1;
TRISAbits.TRISA1 = 1;

//Set Lower 2 Bits to High Impedance
TRISCbits.TRISC0 = 1;
TRISCbits.TRISC1 = 1;
//Set Higher 2 Bits to Output
TRISCbits.TRISC2 = 0;
TRISCbits.TRISC3 = 0;

PORTCbits.RC0 = 0;
PORTCbits.RC1 = 0;
//Provide Ground To X-axis Of Touch Screen
PORTCbits.RC2 = 0;
//Provide Power To X-axis Of Touch Screen
PORTCbits.RC3 = 1;

// configure A/D convertor
OpenADC( ADC_FOSC_32 & ADC_RIGHT_JUST & 
ADC_8ANA_0REF,ADC_CH0 & ADC_INT_OFF );

Delay10TCYx( 5 ); // Delay for 50TCY
ConvertADC(); // Start conversion
while( BusyADC() ); // Wait for completion
result_y = ReadADC(); // Read result
CloseADC();

Electronic – Using a GPIO pin as a power source

Note:
This answer was originally written before we knew anything about the receiver, like its 1.8V power supply. Telaclavo's answer is good. As a more general answer the PNP/MOSFET solution remains; you don't want to power parts of your circuit from a microcontroller's I/O pin.

You don't want to do that! You'd configure the pin as output and make it high to provide power to the GPS receiver, but microcontrollers I/Os can only supply limited current, 25mA for the STM32F205xx (Section 6.2 page 72 of the datasheet), which will be too low for powering your (and any other) GPS receiver (34 to 38 mA, as stated in the datasheet)

Use the I/O pin to drive a PNP transistor which will supply the required current.

enter image description here

Note that using a PNP inverses your logic: a logic low will turn the receiver on.

I would not use an NPN for this. In common emitter it would mean that the receiver's ground is a few hundred mV above ground, and a circuit should have one single ground which is the same for every component. In common collector you would lose too much of your 3.3V power supply.

edit
Wouter would use a MOSFET instead of a BJT, and that's a good alternative. Just make sure you choose a logic-level FET, which will give you enough current at a \$V_{GS}\$ of -3.3V. The Rohm RZE002P02 is a suitable type. It will also have a lower voltage drop if your receiver needs less than about 200mA.

edit 2 (re clabacchio's addition of a datasheet)
This device operates at 1.8V, the STM32 at 3.3V. You can use an LDO with an enable input and control that from your microcontroller. No transistor needed. (Thanks for the suggestion, markrages.) You'll also need level shifters for the data.

Best Answer

Related Solutions

Electronic – Resistive Touch Screens

Electronic – Using a GPIO pin as a power source

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