Two color screens on an Arduino Uno

// Two color screens: a 1.8 inch ST7735 and a 1.3 inch ST7789.
//
//   ST7735   SCK -> 13   SDA -> 11   CS -> 10   A0 -> 9   RESET -> 8   LED -> 5 V
//   ST7789   SCL -> A1   SDA -> A0   DC -> 7    RES -> 6  BLK -> 5 V   CS unwired
//
// Both chips speak the same command set as the ILI9341: a column window, a row
// window, then pixels, with DC saying which bytes are commands. What differs
// is the glass. The ST7789's memory is 240x320 and this panel shows 240x240 of
// it, so rotations that count rows from the bottom start at row 80; and its
// IPS glass shows colors inverted until INVON, which init() sends.
//
// The ST7789 is wired the way the 1.3 inch board comes: no chip select, so it
// listens to every clock on its pins. That is why it has pins of its own here
// (the five-argument constructor is software SPI on any pins). On the ST7735's
// bus it would take that panel's bytes as its own.
//
// Every pixel goes out bit by bit, so an Uno draws a small square quickly and
// a whole screen slowly: draw what moves.

#include <SPI.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ST7735.h>
#include <Adafruit_ST7789.h>

Adafruit_ST7735 small(10, 9, 8);
Adafruit_ST7789 square(-1, 7, A0, A1, 6);  // no CS; DC, SDA, SCL, RES

void setup() {
  small.initR(INITR_BLACKTAB);
  small.fillScreen(ST77XX_BLACK);
  small.setTextColor(ST77XX_GREEN);
  small.setCursor(4, 4);
  small.print("ST7735 128X160");

  square.init(240, 240, SPI_MODE3);
  square.setRotation(2);
  square.fillRect(0, 0, 240, 40, ST77XX_BLUE);
  square.setTextColor(ST77XX_WHITE);
  square.setTextSize(2);
  square.setCursor(8, 12);
  square.print("ST7789 IPS");
}

void loop() {
  // A block that walks around each screen, in a color that changes as it goes.
  static uint8_t step = 0;
  uint16_t color = Adafruit_ST7789::color565((uint8_t)(step * 16), 255 - step * 16, 128);
  small.fillRect((int16_t)(8 + (step % 8) * 14), 40, 12, 12, color);
  square.fillRect((int16_t)(10 + (step % 8) * 28), 120, 24, 24, color);
  step = (uint8_t)((step + 1) % 16);
  delay(100);
}