LCD1602 in 8-bit mode on pins 2 to 12, on an Arduino Uno

// A 16x2 LCD in 8-bit mode, with no library: every byte by hand.
//
//   LCD RS -> pin 12    RW -> pin 11    E -> pin 2
//   LCD D0 to D7 -> pins 3 to 10
//   LCD VSS, K -> GND   VDD, A -> 5V   V0 -> GND (full contrast)
//
// The display's controller (an HD44780) takes one byte at a time on D0 to
// D7. RS says what the byte is: LOW for a command (clear, move the cursor),
// HIGH for a character to print. RW LOW means the board is writing. A short
// pulse on E tells the controller to take the byte.
//
// The LiquidCrystal library does all this, usually over four data lines to
// save pins. Doing it by hand over eight shows what the library hides.

const int RS_PIN = 12;
const int RW_PIN = 11;
const int E_PIN = 2;
const int DATA_PINS[8] = {3, 4, 5, 6, 7, 8, 9, 10};  // D0 to D7

void lcdSend(byte value, bool isText) {
  digitalWrite(RS_PIN, isText ? HIGH : LOW);
  digitalWrite(RW_PIN, LOW);
  for (int bit = 0; bit < 8; bit++) {
    digitalWrite(DATA_PINS[bit], (value >> bit) & 1);
  }
  digitalWrite(E_PIN, HIGH);
  delayMicroseconds(1);
  digitalWrite(E_PIN, LOW);  // the controller takes the byte here
  delayMicroseconds(50);     // most commands are done in 40 us
}

void lcdCommand(byte command) {
  lcdSend(command, false);
  if (command <= 0x03) delay(2);  // clear and home take longer
}

void lcdPrint(const char *text) {
  while (*text) lcdSend(*text++, true);
}

// Move the cursor: row 0 starts at address 0x00, row 1 at 0x40.
void lcdGoTo(byte column, byte row) {
  lcdCommand(0x80 | (row ? 0x40 : 0x00) | column);
}

void setup() {
  pinMode(RS_PIN, OUTPUT);
  pinMode(RW_PIN, OUTPUT);
  pinMode(E_PIN, OUTPUT);
  for (int bit = 0; bit < 8; bit++) pinMode(DATA_PINS[bit], OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: 16x2 LCD in 8-bit mode, no library");

  delay(50);           // let the display power up
  lcdCommand(0x38);    // 8-bit interface, two lines, 5x8 dots
  delay(5);
  lcdCommand(0x38);
  lcdCommand(0x38);
  lcdCommand(0x0C);    // display on, no cursor, no blink
  lcdCommand(0x06);    // move right after each character
  lcdCommand(0x01);    // clear

  lcdGoTo(1, 0);
  lcdPrint("Eight data pins");
  lcdGoTo(1, 1);
  lcdPrint("and no library");
  delay(3000);
  lcdCommand(0x01);
  lcdGoTo(0, 0);
  lcdPrint("Hello, world!");
}

void loop() {
  // The seconds since reset, as text: digits are the characters '0' to '9'.
  unsigned long seconds = millis() / 1000;
  char digits[11];
  int n = 0;
  do {
    digits[n++] = '0' + seconds % 10;
    seconds /= 10;
  } while (seconds > 0);
  lcdGoTo(0, 1);
  lcdPrint("Up ");
  while (n > 0) lcdSend(digits[--n], true);
  lcdPrint(" s");
  delay(250);
}