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UART Loopback

This sample demonstrates basic data read and write operations over a UART. The application creates a loopback interface that immediately echoes any data read from the receive (RX) buffer to the transmit (TX) buffer.


UART Loopback sample demo

(Watch the demo on YouTube)


  • Android Things compatible board with an available UART port
  • Android Studio 2.2+
  • FTDI TTL-232R or compatible USB-TTL converter


Schematics for Raspberry Pi 3

NOTE: Raspberry Pi 3 shares the UART pins between multiple ports, including the serial debugging console. Refer to the mode matrix for more details.

Check in the USB TTL cable's documentation for the TX and RX wire colors:

  • connect the cable TX wire to the board RX pin
  • connect the cable RX wire to the board TX pin

Build and install

  1. On Android Studio, click on the "Run" button.

If you prefer to run on the command line, type

./gradlew installDebug
adb shell am start com.example.androidthings.loopback/.LoopbackActivity
  1. Plug the USB-TTL converter into a host PC, start your favorite terminal program (minicom, screen, RealTerm, etc.), and connect to the USB-TTL port at 115200 baud.

  2. Type characters into the terminal and observe them echo back to you from the device.


Copyright 2016 The Android Open Source Project, Inc.

Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


Echo received characters over a UART with Android Things





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