Gpio - Juno: Java for Arduino

Gpio is the board's set of pins. Get it once with ArduinoUno.getGpio() or ArduinoNano.getGpio(), then set a pin as an output, write it high or low, or read an input. DigitalOutput is a lighter handle for a pin you only switch on and off.

Method	Argument	Notes
`pinMode(pin, mode)`	a pin, and `Gpio.getInput()`, `getOutput()` or `getInputPullup()`	sets the direction of a pin
`digitalWrite(pin, high)`	a pin and a `boolean`	drives the pin high (`true`) or low (`false`)
`digitalRead(pin)`	a pin	returns `true` when the pin is high
`analogRead(pin)`	an analog pin, such as `Gpio.getA0()`	returns the ADC reading
`analogWrite(pin, value)`	a pin and a value	PWM output on a pin that supports it
`getBuiltinLed()`	none	the pin of the board's onboard LED

Pins are portable numbers, not the chip's own GPIO numbers. Gpio.getD0() through Gpio.getD13() return 0 to 13, and Gpio.getA0() through Gpio.getA5() return 14 to 19. The compiler translates these numbers to the linked board's pins, so the same program runs on a UNO R4 WiFi or a Nano ESP32 without changes.

DigitalOutput.of(pin) sets the pin as an output and returns a handle with high() and low().

Blink

Blink turns the board's onboard LED on and off forever: on for 500 milliseconds, off for 500 milliseconds, one full blink each second. It is the first program to run on any board, because it needs no wiring:

import io.github.jabrena.juno.hardware.ArduinoUno;
import io.github.jabrena.juno.hardware.gpio.DigitalOutput;
import io.github.jabrena.juno.util.Delay;

public class Blink {

    public static void main(String[] args) {

        DigitalOutput led = DigitalOutput.of(ArduinoUno.getGpio().getBuiltinLed());

        while (true) {
            led.high();
            Delay.millis(500);

            led.low();
            Delay.millis(500);
        }
    }
}

Step by step:

ArduinoUno.getGpio().getBuiltinLed() asks the linked board which pin its LED is on. The same line works on every board, so the program names no pin.
DigitalOutput.of(...) configures that pin as an output and returns a handle to it.
The loop sets the pin high, waits with Delay, sets it low and waits again. Creating the handle happens once, before the loop, so the loop allocates nothing.

Build, verify and upload it with the Maven plugin. juno:verify compiles the generated sketch with the Arduino toolchain without touching the board:

./mvnw -f juno-examples/pom.xml compile juno:verify \
  -Djuno.main=io.github.jabrena.juno.api.hardware.gpio.Blink \
  -Djuno.board=arduino-uno-r4-wifi

./mvnw -f juno-examples/pom.xml compile juno:upload \
  -Djuno.main=io.github.jabrena.juno.api.hardware.gpio.Blink \
  -Djuno.board=arduino-uno-r4-wifi

Blink prints nothing over Serial, so there is no output to read with juno:monitor. The LED is the result.

Reading an input

digitalRead and analogRead need a pin that is configured as an input. A button between a digital pin and ground, with the internal pull-up enabled, reads false while the button is pressed:

Gpio gpio = ArduinoUno.getGpio();
gpio.pinMode(Gpio.getD2(), Gpio.getInputPullup());

boolean pressed = !gpio.digitalRead(Gpio.getD2());

analogRead on an analog pin returns the ADC value, which is a number from the converter rather than a voltage:

int light = gpio.analogRead(Gpio.getA0());

Troubleshooting

The LED does not blink: check the board name in -Djuno.board matches the board you plugged in, and that juno:upload finished without an error.
A digitalRead always returns the same value: the pin was not set as an input with pinMode first, or a button is not wired between the pin and ground.