STM32 and 4-wire resistive touch screen - c

Question

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.

Answer 1

Between selecting the touch event and reading the touch position, you must reconfigure the pins. The following is from an NXP application note (AN10675), but applies to any microcontroller and can be adapted to STM32:
The outputs are set to provide a voltage source for the analogue measurement of the effective voltage divider formed by the touch position:
On the STM32 it is of course important not to use the GPIO configuration lock feature on these pins as you need three different configurations to read a single touch position event.