MAX7219 scrolling text display on an Arduino Uno

// MAX7219 scrolling text: a 4-in-1 dot matrix module (four 8x8 matrices,
// 32 by 8 LEDs) scrolls a message right to left with MD_Parola, the library
// most scrolling signs use. Type a new message in the serial monitor and
// press Enter, and it takes over at the start of the next pass.
//
//   MAX7219 module   VCC 5V, GND GND
//                    DIN pin 11 (MOSI), CLK pin 13 (SCK), CS pin 10
//
// MD_Parola sits on top of MD_MAX72xx, which talks to the four MAX7219 chips
// over SPI. displayText() sets the message up once; displayAnimate() never
// waits, it moves the text one column each time SCROLL_MS has passed, so
// loop() stays free to read the serial monitor in between. When a pass ends,
// displayAnimate() returns true and displayReset() starts the next one.
//
// HARDWARE_TYPE says how the matrices on a real board are wired to their
// chips. The blue 4-in-1 boards sold for scrolling text are FC16_HW; if your
// text comes out mirrored or upside down, try PAROLA_HW or GENERIC_HW.
//
// Things to change: MAX_DEVICES for a longer chain (8 for two boards end to
// end), SCROLL_MS for the speed (smaller is faster), and setIntensity() for
// the brightness, 0 to 15. A real module on full brightness can pull close
// to an amp, so keep it low on USB power.

#include <MD_Parola.h>
#include <MD_MAX72xx.h>
#include <SPI.h>

#define HARDWARE_TYPE MD_MAX72XX::FC16_HW
#define MAX_DEVICES 4
#define CS_PIN 10

const uint16_t SCROLL_MS = 40;  // time per column, in milliseconds
const int MAX_TEXT = 64;

MD_Parola display(HARDWARE_TYPE, CS_PIN, MAX_DEVICES);

char message[MAX_TEXT + 1] = "Hello from Mokxi!";
char typed[MAX_TEXT + 1];
int typedLength = 0;
bool waiting = false;  // a new message is ready for the next pass

void setup() {
  Serial.begin(9600);
  display.begin();
  display.setIntensity(2);
  display.displayClear();
  display.displayText(message, PA_LEFT, SCROLL_MS, 0, PA_SCROLL_LEFT, PA_SCROLL_LEFT);
  Serial.print("Scrolling: ");
  Serial.println(message);
  Serial.println("Type a new message and press Enter.");
}

void loop() {
  // Collect what is typed, one character at a time, until Enter.
  while (Serial.available()) {
    char c = Serial.read();
    if (c == '\n') {
      if (typedLength > 0) {
        typed[typedLength] = '\0';
        waiting = true;
        Serial.print("Next pass: ");
        Serial.println(typed);
      }
      typedLength = 0;
    } else if (c >= ' ' && typedLength < MAX_TEXT) {
      // Printable characters only: a carriage return, a tab or any other
      // control character has no shape in the font.
      typed[typedLength++] = c;
    }
  }

  // The end of a pass: start the next one, with the new message if there is one.
  if (display.displayAnimate()) {
    if (waiting) {
      strcpy(message, typed);
      waiting = false;
      Serial.print("Scrolling: ");
      Serial.println(message);
    }
    display.displayReset();
  }
}