Touch paint on an Arduino Uno

// Touch paint: draw on a 2.8 inch ILI9341 with its XPT2046 touch panel.
//
//   display   CS -> 10   DC -> 9   RST -> 8   MOSI -> 11   SCK -> 13   LED -> 5 V
//   touch     T_CS -> 7   T_CLK -> A2   T_DIN -> A0   T_DO -> A1
//
// Press on the picture while it runs, and drag.
//
// Two chips on one module, each an SPI device with its own chip select. They
// could share one bus, since each only listens while its own select is low;
// here the touch controller has a bus of its own on A0 to A2, an SPIClass
// made on those pins, so the two sets of wires never meet.
// Each reading is a control byte and sixteen clocks: the start bit, the
// channel, then twelve bits of result in bits 14 to 3 (the >> 3). Pressure is
// 4095 + Z1 - Z2, large for a press and near nothing for none.
//
// The readings are raw counts, about 200 at one edge of the picture and 3900
// at the other, so map() turns them into pixels. With the display and the
// touch at the same rotation, a bigger x is further right.
//
// The row of colors along the top is the palette: touch one to pick it.

#include <SPI.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ILI9341.h>
#include <XPT2046_Touchscreen.h>

Adafruit_ILI9341 tft(10, 9, 8);
SPIClass touchBus(A2, A1, A0, 7);  // SCK, MISO, MOSI, SS
XPT2046_Touchscreen ts(7);

const uint16_t PALETTE[] = {ILI9341_RED, ILI9341_YELLOW, ILI9341_GREEN,
                            ILI9341_CYAN, ILI9341_BLUE,  ILI9341_WHITE};
const int SWATCH = 40;
uint16_t ink = ILI9341_YELLOW;

void setup() {
  tft.begin();
  tft.setRotation(1);
  tft.fillScreen(ILI9341_BLACK);
  for (int i = 0; i < 6; i++) tft.fillRect(i * SWATCH, 0, SWATCH, 20, PALETTE[i]);
  ts.begin(touchBus);
  ts.setRotation(1);
}

void loop() {
  if (!ts.touched()) return;
  TS_Point p = ts.getPoint();
  int x = map(p.x, 200, 3900, 0, tft.width());
  int y = map(p.y, 200, 3900, 0, tft.height());
  if (y < 20) {
    if (x >= 0 && x < 6 * SWATCH) ink = PALETTE[x / SWATCH];
    return;
  }
  tft.fillRect(x - 2, y - 2, 5, 5, ink);
}