LCD Keypad Shield

LcdKeypadShield drives the common "LCD Keypad Shield" — a 16x2 HD44780-compatible character LCD in 4-bit mode, plus 5 buttons wired through a single resistor ladder into one analog pin — using its standard pinout, as used by e.g. dzindra/lcdkeypad.

Unlike the LED matrix, this shield needs no new compiler intrinsic: every operation is built entirely from Gpio pin operations and Delay, so it works today on any board Juno's ASM backend targets.

Wiring

The driver assumes the shield's standard pinout and needs no additional configuration:

Function	Pin
LCD RS	digital 8
LCD Enable	digital 9
LCD data (D4-D7)	digital 4-7
Backlight	digital 10
Buttons (analog ladder)	A0

API

import io.github.jabrena.juno.api.lcd.LcdKeypadShield;

LcdKeypadShield.begin();                 // pin setup + HD44780 4-bit init + backlight on
LcdKeypadShield.clear();                 // clear display, cursor to (0, 0)
LcdKeypadShield.home();                  // cursor to (0, 0), keep contents
LcdKeypadShield.setCursor(0, 1);         // column 0-15, row 0-1
LcdKeypadShield.print("Hello");          // print text at the cursor, no wrapping
LcdKeypadShield.print(42);               // print an int in decimal
LcdKeypadShield.backlight(false);        // turn the backlight off (or on)

int button = LcdKeypadShield.readButton(); // undebounced read of the currently pressed button

readButton() returns one of LcdKeypadShield.NONE, RIGHT, UP, DOWN, LEFT, or SELECT. COLUMNS (16) and ROWS (2) describe the character grid. Reads are undebounced — a program that needs a clean single "press" event should only act when the value changes from NONE, as the example below does.

Example: showing which button is pressed

import io.github.jabrena.juno.annotations.ArduinoUnoR4WiFi;
import io.github.jabrena.juno.annotations.Board;
import io.github.jabrena.juno.api.Delay;
import io.github.jabrena.juno.api.lcd.LcdKeypadShield;

@Board(ArduinoUnoR4WiFi.class)
public final class LcdKeypadDemo {
    public static void main(String[] args) {
        LcdKeypadShield.begin();
        LcdKeypadShield.print("Press a button");

        int lastButton = -1;
        while (true) {
            int button = LcdKeypadShield.readButton();
            if (button != lastButton) {
                LcdKeypadShield.setCursor(0, 1);
                String label = "NONE  ";
                if (button == LcdKeypadShield.RIGHT) {
                    label = "RIGHT ";
                } else if (button == LcdKeypadShield.UP) {
                    label = "UP    ";
                } else if (button == LcdKeypadShield.DOWN) {
                    label = "DOWN  ";
                } else if (button == LcdKeypadShield.LEFT) {
                    label = "LEFT  ";
                } else if (button == LcdKeypadShield.SELECT) {
                    label = "SELECT";
                }
                LcdKeypadShield.print(label);
                lastButton = button;
            }
            Delay.millis(50);
        }
    }
}

This is juno-examples/.../lcd/LcdKeypadDemo.java. Build, flash, and run it with juno-maven-plugin (see docs/JUNO-MAVEN-PLUGIN.md):

./mvnw -f juno-examples/pom.xml compile juno:upload \
  -Djuno.main=io.github.jabrena.juno.api.lcd.LcdKeypadDemo

Note that each printed label is padded to a fixed width ("NONE ", "SELECT") so a shorter new label fully overwrites a longer previous one on the fixed 16-column display — print does not clear the rest of the row.

A larger example: Pomodoro

juno-examples/.../lcd/pomodoro implements a small Pomodoro timer entirely on the shield: ConfigurePomodoroView lets the buttons set a duration, RunPomodoro counts it down while showing elapsed time, and TimeUpView signals completion — a good reference for a multi-screen, button-driven program built only from LcdKeypadShield and Delay.

Notes

• backlight(true) floats the backlight pin (INPUT) rather than driving it HIGH, because the shield's backlight LED has no current-limiting resistor on some board revisions and driving the pin HIGH can pull the shared 5V rail down hard enough to keep the LCD from initializing at all.
• The HD44780 init sequence and every command/data write include generous settle delays; this keeps the shield reliable under voltage sag from other onboard activity (for example WiFi radio bursts on the UNO R4 WiFi) at the cost of a few extra microseconds per operation — never noticeable for a display updated a few times a second.
• print(String) writes each character with charAt(i), so a compile-time literal or any runtime String supporting length()/charAt(int) (see docs/FEATURES.md) both work, unlike Serial.print(String), which is literal-only.