XPT2046_Touchscreen: reading a resistive touch panel

// Touch paint: draw on the Cheap Yellow Display with a finger.
//
// The screen is TFT_eSPI with the board's setup. The touch controller, an
// XPT2046, is not on the screen's bus: the board wires it to GPIO 25 (CLK),
// 32 (DIN), 39 (DO) and 33 (CS), so it gets an SPI bus of its own, the way
// the CYD tutorials do it. Press the screen while the sketch runs to draw.

#include <SPI.h>
#include <TFT_eSPI.h>
#include <XPT2046_Touchscreen.h>

#define XPT2046_IRQ 36
#define XPT2046_MOSI 32
#define XPT2046_MISO 39
#define XPT2046_CLK 25
#define XPT2046_CS 33

SPIClass touchscreenSPI = SPIClass(VSPI);
XPT2046_Touchscreen touchscreen(XPT2046_CS, XPT2046_IRQ);
TFT_eSPI tft = TFT_eSPI();

const int kWidth = 320;
const int kHeight = 240;
const uint16_t kInks[4] = {TFT_YELLOW, TFT_CYAN, TFT_MAGENTA, TFT_GREEN};
int ink = 0;

void drawPalette()
{
    for (int i = 0; i < 4; i++) {
        tft.fillRect(i * 40, 0, 40, 30, kInks[i]);
    }
    tft.fillRect(160, 0, 160, 30, TFT_DARKGREY);
    tft.setTextColor(TFT_WHITE, TFT_DARKGREY);
    tft.drawCentreString("clear", 240, 8, 2);
}

void setup()
{
    Serial.begin(115200);
    touchscreenSPI.begin(XPT2046_CLK, XPT2046_MISO, XPT2046_MOSI, XPT2046_CS);
    touchscreen.begin(touchscreenSPI);
    touchscreen.setRotation(1);

    tft.init();
    tft.setRotation(1);
    tft.fillScreen(TFT_BLACK);
    drawPalette();
}

void loop()
{
    if (!touchscreen.tirqTouched() || !touchscreen.touched()) return;
    TS_Point p = touchscreen.getPoint();
    // The raw readings run about 200 to 3700 across the glass.
    int x = map(p.x, 200, 3700, 0, kWidth - 1);
    int y = map(p.y, 240, 3800, 0, kHeight - 1);
    x = constrain(x, 0, kWidth - 1);
    y = constrain(y, 0, kHeight - 1);
    if (y < 30) {
        if (x < 160) {
            ink = x / 40;
        } else {
            tft.fillRect(0, 30, kWidth, kHeight - 30, TFT_BLACK);
        }
        delay(150);
        return;
    }
    tft.fillCircle(x, y, 3, kInks[ink]);
}