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You can use a phone as a wireless serial monitor when your board runs firmware that exposes a UART-style Bluetooth Low Energy (BLE) GATT service. With Nordic’s UART Service (NUS), a BLE client app can send text or commands to the device and display its UART output. BLE pairing by itself is not enough: the firmware and app must support matching services and characteristics.
What you need
- A BLE-capable board or adapter: Use a development board or UART-to-BLE adapter running firmware that exposes NUS or another UART-style GATT service. Nordic’s Peripheral UART examples use nRF52-series development kits. A generic BLE advertisement alone does not provide a serial connection; the client needs the expected writable and notification-capable characteristics.
- A BLE GATT client app: Nordic’s nRF Connect for Mobile can scan for BLE devices, connect, inspect services, and show a log of exchanged data.
- Matching service details: The app and firmware must agree on the service UUID and on which characteristics accept writes and send notifications. NUS is one documented option, not the only way to build a BLE UART bridge.
Connect and exchange serial data
Nordic’s Peripheral UART sample uses NUS to pass data between a UART connection and a BLE connection, emulating a serial port over BLE. The phone acts as the BLE client; the board runs the peripheral firmware.
- Install and open nRF Connect for Mobile, then scan for nearby BLE devices.
- Select your board and connect. Nordic’s example advertises as “Nordic_UART.”
- Inspect the discovered services and open Nordic UART Service (NUS).
- To send data from the phone to the board, write text to the UART RX characteristic. In Nordic’s documented test, the phone sends
0123456789. - To receive board output, enable notifications on the UART TX characteristic. Text entered through the board’s UART terminal then appears in the phone app’s log.
Check the UART and BLE settings
Match the host UART configuration
The board’s UART connection still has its own settings, including baud rate. Set the connected host or terminal to match the firmware and board configuration. Nordic lists 115200 baud as the default for UART_0 on the Thingy:91; that is a device-specific example, not a universal BLE speed or a setting for every board.
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Verify both directions
Phone-to-device traffic depends on a writable RX characteristic. Device-to-phone traffic depends on TX notifications being implemented and enabled in the app. If you can send commands but see no output, check the TX characteristic’s notification setting first.
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Account for security settings
Pairing or bonding may be required if the firmware is configured to use it. Nordic’s procedure supports optional pairing or bonding with MITM protection when enabled in the firmware.
Troubleshoot connection and missing data
- The device does not appear in the scan: Confirm that it is powered on, advertising, and running the intended peripheral firmware.
- The service is missing or looks different: Verify that the firmware exposes the service UUID your app expects. A vendor-specific UART service may use a different UUID and characteristic layout from NUS.
- The phone can connect but cannot send: Check that the RX characteristic exists and accepts writes, then write to that characteristic rather than another service value.
- The board receives commands but the phone shows no output: Confirm that the board is sending data over its UART path and enable notifications on the TX characteristic.
- Text is garbled or the host terminal cannot communicate: Check the host UART configuration, including baud rate, against the board’s settings.
Choose a compatible BLE UART implementation
If you are selecting between a development board and an adapter, compare the properties that determine whether they will work with your app and equipment:
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| Check | What to verify |
|---|---|
| Service compatibility | Whether the device exposes NUS or a vendor-specific service, and whether its UUIDs match the client’s expectations. |
| Data direction | Whether the firmware supports both writes from the phone to RX and notifications from TX to the phone. |
| Reliability and throughput | How the implementation handles MTU, notifications, buffering, and reconnection. These details depend on the device and firmware. |
| Security | Whether pairing or bonding is required and whether MITM protection is supported. |
| UART host integration | Whether the board or adapter fits your UART voltage levels and connector, and which baud configuration it requires. |
Why the phone’s Bluetooth settings are not enough
Ordinary Bluetooth settings are not a terminal for arbitrary BLE GATT characteristics. For a custom UART service, use a GATT client to scan, connect, inspect the service, write to RX, and subscribe to TX notifications.
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