M5Stack Cardputer

The M5Stack Cardputer is a pocket computer built around an Espressif ESP32-S3, with its own keyboard, color display, speaker, microSD slot and battery. Its CPU is an Xtensa LX7, so Juno compiles for it with the same Xtensa backend as the Nano ESP32. The M5StackCardputer device facade is the entry point for its peripherals.

Core specification

Characteristic	M5Stack Cardputer
Microcontroller	Espressif ESP32-S3
CPU architecture	Dual-core 32-bit Xtensa LX7
CPU clock	240 MHz
Program flash	8 MB
SRAM	512 kB
Logic and operating voltage	3.3 V
USB connector	USB-C (native USB)
Display	240 × 135 pixel color
Built-in hardware	Keyboard, speaker, microSD slot, battery
Arduino CLI FQBN	`m5stack:esp32:m5stack_cardputer`
Juno board id	`m5stack-cardputer`

What Juno supports

Feature	Status	Notes	Example
`Delay`, `Serial`	✓	`Serial` over the native USB port, at 115200 baud.
Classes, interfaces and inheritance	✓		`Inheritance`, `Interfaces`
`BigInteger`, `BigDecimal`, `MathContext`	✓
`SdCard`	✓	Built-in microSD slot through `M5StackCardputer.getSd()`.	`M5CardputerSd`
Wi-Fi station and access point	✓	`M5StackCardputer.getWifi()`; credentials can come from the microSD card.	`M5CardputerWifi`, `M5CardputerWifiStatusSD`
SNTP clock (`LocalDateTime.now()`)	✓	UTC over SNTP, once Wi-Fi is up; the server can be set on the card with `sntp.host` and `sntp.port`.	`M5CardputerSntpTime`
`HttpClient` (HTTP and HTTPS)	✓	HTTPS with a trusted certificate set by the program.	`MailboxCardputer`
`HttpServer` (HTTP and HTTPS)	✓	`HttpServer.beginTls` serves HTTPS on the station address.	`CardputerHttpsServerSD`
Display	✓	240 × 135 RGB565 through `M5StackCardputer.getDisplay()`.	`M5CardputerDisplay`
Keyboard	✓	`M5StackCardputer.getKeyboard()`, polled without allocation.	`M5CardputerKeyboard`
Speaker	✓	`M5StackCardputer.getAudio()`: `tone`, `setVolume`, `stop`.	`ZeldaOcarinaCardputer`
`@Watchdog`	✓	ESP32-S3 task watchdog; the timeout is rounded up to whole seconds.
Battery	✓	`M5StackCardputer.getBattery()`: estimated level and voltage.	`M5CardputerBatteryLevel`
USB HID mouse	✓	`M5StackCardputer.getMouse()` through TinyUSB; shares the USB port with `Serial`.	`M5RatonLoco`

Features

SD card

M5StackCardputer.getSd() returns the Cardputer's built-in microSD socket. Calling begin() uses its standard SPI wiring: chip select GPIO12, MOSI GPIO14, MISO GPIO39, and clock GPIO40.

SdCard sd = M5StackCardputer.getSd();
if (sd.begin()) {
    Serial.println("SD card ready");
}

Wi-Fi

M5StackCardputer.getWifi() returns the Cardputer's built-in ESP32-S3 radio. Keep the network's credentials on the microSD card rather than in the source: put them in application.properties at the card's root,

wifi.ssid=your-network
wifi.password=your-password

then read the file with java.util.Properties and join the network:

SdCard sd = M5StackCardputer.getSd();
Wifi wifi = M5StackCardputer.getWifi();

if (sd.begin()) {
    try (InputStream file = sd.open("application.properties")) {
        if (file != null) {
            Properties properties = new Properties();
            properties.load(file);
            wifi.begin(properties.getProperty("wifi.ssid"), properties.getProperty("wifi.password"));
        }
    }
}
while (wifi.status() != Wifi.STATUS_CONNECTED) {
    Delay.millis(1000);
}

M5CardputerWifiStatusSD is the complete program: it joins the network and shows the connection status on the display.

./mvnw -f juno-examples/pom.xml compile juno:upload \
  -Djuno.main=io.github.jabrena.juno.api.hardware.net.wifi.M5CardputerWifiStatusSD

Display

M5StackCardputer.getDisplay() returns the Cardputer's 240 by 135 pixel color display. Colors use RGB565 values, with common colors available as constants and Display.color(red, green, blue) for custom colors. Call begin() once before drawing.

Display display = M5StackCardputer.getDisplay();
display.begin();
display.fillScreen(Display.BLACK);
display.fillRect(8, 8, display.width() - 16, 4, Display.BLUE);
display.setCursor(12, 24);
display.setTextColor(Display.GREEN, Display.BLACK);
display.setTextSize(2);
display.println("Hello, Juno!");

The display interface also provides height(), drawPixel(...), print(...), and println(...). Applications can receive a mock Display in desktop unit tests.

Keyboard

M5StackCardputer.getKeyboard() returns the Cardputer's built-in keyboard. Initialize it once, call update() in the main loop, and inspect the state after isChanged() reports a transition.

Keyboard keyboard = M5StackCardputer.getKeyboard();
keyboard.begin();
while (true) {
    keyboard.update();
    if (keyboard.isChanged() && keyboard.isPressed()) {
        for (int index = 0; index < keyboard.keyCount(); index++) {
            Serial.println(keyboard.keyAt(index));
        }
    }
    Delay.millis(20);
}

keyAt(...) returns printable character codes or Keyboard.BACKSPACE, Keyboard.TAB, and Keyboard.ENTER. isKeyPressed(...) checks one character or control key. Shift, Control, Alt, Fn, and Option have dedicated state methods, so no array or collection needs to be allocated while polling.

Speaker

M5StackCardputer.getAudio() returns the built-in speaker. tone(frequencyHz, durationMillis) plays a note, setVolume(...) sets the loudness and stop() silences it.

Audio audio = M5StackCardputer.getAudio();
audio.setVolume(128);
audio.tone(440, 500);

ZeldaOcarinaCardputer plays a melody on it.

Battery

M5StackCardputer.getBattery() reports the built-in battery's estimated charge level in percent and its voltage in millivolts.

Battery battery = M5StackCardputer.getBattery();
Serial.print(battery.levelPercent());
Serial.println("%");

USB HID mouse

The Cardputer's ESP32-S3 can act as a USB HID mouse through its core's TinyUSB stack. Call begin() once, then move(x, y) moves the computer's pointer by that many pixels:

Mouse mouse = M5StackCardputer.getMouse();
mouse.begin();

while (true) {
    mouse.move(50, 0);   // right
    Delay.millis(1000);
    mouse.move(0, 50);   // down
    Delay.millis(1000);
    mouse.move(-50, 0);  // left
    Delay.millis(1000);
    mouse.move(0, -50);  // up
    Delay.millis(1000);
}

Once uploaded, the program keeps moving the pointer of the computer it is plugged into; disconnect the board or upload another program to stop it.

M5RatonLoco moves the host pointer around a square and reports each side over the USB serial console. The native TinyUSB device may receive a new serial-port path after flashing: use arduino-cli board list and pass that new /dev/cu.usbmodem... path as -Djuno.port if the monitor does not attach.

Build and run

Run juno:install-deps once to install the M5Stack ESP32 board package and the official M5Cardputer library along with the other supported board cores and libraries. Then compile and upload an example, for instance the battery report:

./mvnw -f juno-examples/pom.xml compile juno:upload \
  -Djuno.main=io.github.jabrena.juno.api.hardware.battery.M5CardputerBatteryLevel \
  -Djuno.board=m5stack-cardputer

Open the serial monitor in another terminal:

./mvnw -f juno-examples/pom.xml juno:monitor \
  -Djuno.board=m5stack-cardputer

The monitor uses 115200 baud by default, matching Serial.begin(BaudRate.BAUD_115200).