pxt-calliope/docs/reference/bluetooth/uart-write.md

UART Write

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For another device like a smartphone to use any of the Bluetooth "services" which the micro:bit has, it must first be paired with the micro:bit. Once paired, the other device may connect to the micro:bit and exchange data relating to many of the micro:bit's features.

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The Bluetooth UART service allows another device such as a smartphone to exchange any data it wants to with the micro:bit, in small chunks.

With the Bluetooth UART service running, this block allows a micro:bit to send data to a Bluetooth connected device.

bluetooth.uartWrite("");

Example: Starting the Bluetooth UART service and then sending "HELLO" whenever button A is pressed and another device has connected over Bluetooth

let connected = 0;
bluetooth.onBluetoothConnected(() => {
    basic.showString("C");
    connected = 1;
});
bluetooth.onBluetoothDisconnected(() => {
    basic.showString("D");
    connected = 0;
});
bluetooth.startUartService();
input.onButtonPressed(Button.A, () => {
    if (connected == 1) {
        bluetooth.uartWrite("HELLO");
    }
});

Video - UART service guessing game

https://www.youtube.com/watch?v=PgGeWddMAZ0

Advanced

For more advanced information on the micro:bit Bluetooth UART service including information on using a smartphone, see the Lancaster University micro:bit runtime technical documentation

See also

Bluetooth SIG, Bluetooth on micro:bit resources

microbit-bluetooth