Matrix ball on an Arduino Uno

// Matrix ball: a dot bouncing round an 8x8 LED matrix on a MAX7219.
//
// Wiring, module to board:
//   DIN -> pin 11
//   CS  -> pin 10   (marked LOAD on some boards)
//   CLK -> pin 13
//   VCC -> 5 V, GND -> GND
//
// The MAX7219 holds eight digit registers and scans them out itself. On a
// matrix board those eight registers are the eight rows and the eight segment
// outputs are the columns, so writing a row register *is* drawing a row. The
// driver in mokxi_max7219.h keeps the eight bytes in RAM, lets the sketch poke
// pixels into them, and pushes all eight out on show().
//
// Every register comes up zero at power-on (shut down, one row scanned,
// minimum brightness), which is why nothing happens on a real board until
// begin() writes Shutdown, Scan Limit and Intensity.
//
// The ball is 2x2 and moves one pixel a frame. When it hits a wall it bounces
// and that wall lights for a frame, so the collision is something you can see
// rather than something you have to believe.

#include "mokxi_max7219.h"

const uint8_t DIN_PIN = 11;
const uint8_t CS_PIN = 10;
const uint8_t CLK_PIN = 13;

Max7219 matrix(DIN_PIN, CS_PIN, CLK_PIN, 1);

// Top left of the 2x2 ball, so it runs 0..6 on both axes.
const int8_t BALL = 2;
const int8_t LIMIT = 8 - BALL;

int8_t x = 0;
int8_t y = 0;
int8_t dx = 1;
int8_t dy = 1;

// Which wall was hit last frame: one bit each, top right bottom left.
uint8_t flash = 0;

const int FRAME_MS = 90;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Uno: a ball round an 8x8 matrix");
  matrix.begin(8);
}

void loop() {
  x = (int8_t)(x + dx);
  y = (int8_t)(y + dy);
  flash = 0;

  if (x <= 0) {
    x = 0;
    dx = 1;
    flash |= 0x08;  // left
  } else if (x >= LIMIT) {
    x = LIMIT;
    dx = -1;
    flash |= 0x02;  // right
  }
  if (y <= 0) {
    y = 0;
    dy = 1;
    flash |= 0x01;  // top
  } else if (y >= LIMIT) {
    y = LIMIT;
    dy = -1;
    flash |= 0x04;  // bottom
  }

  matrix.clear();
  if (flash & 0x01) matrix.fillRect(0, 0, 8, 1, true);
  if (flash & 0x02) matrix.fillRect(7, 0, 1, 8, true);
  if (flash & 0x04) matrix.fillRect(0, 7, 8, 1, true);
  if (flash & 0x08) matrix.fillRect(0, 0, 1, 8, true);
  matrix.fillRect(x, y, BALL, BALL, true);
  matrix.show();

  // A bounce is also a flicker of brightness, which costs one packet.
  matrix.setIntensity(flash ? 15 : 8);

  delay(FRAME_MS);
}