HW flow control (CTS+RTS) should always be used with the nRF51822. The UART peripheral block is designed in a way that more or less requires HW flow control for reliable UART communication, with a very small internal buffer and tight timing limitations due to the SoftDevice architecture (Nordic's proprietary Bluetooth Low Energy stack).

Power Pins

  • VIN: This is the power supply for the module, supply with 3.3-16V power supply input. This will be regulated down to 3.3V to run the chip
  • GND: The common/GND pin for power and logic


  • TXO - This is the UART Transmit pin out of the breakout (Bluefruit LE --> MCU), it's at 3.3V logic level.
  • RXI - This is the UART Receive pin into the breakout (MCU --> Bluefruit LE). This has a logic level shifter on it, you can use 3-5V logic.
  • CTS - Clear to Send hardware flow control pin into the the breakou (MCU --> Bluefruit LE). Use this pin to tell the Bluefruit that it can send data back to the microcontroller over the TXO pin. This pin is pulled high by default and must be set to ground in order to enable data transfer out! If you do not need hardware flow control, tie this pin to ground it is a level shifted pin, you can use 3-5V logic
  • RTS - Read to Send flow control pin out of the module (Bluefruit LE --> MCU). This pin will be low when its fine to send data to the Bluefruit. In general, at 9600 baud we haven't seen a need for this pin, but you can watch it for full flow control! This pin is 3.3V out

Other Pins

  • MOD: Mode Selection. The Bluefruit has two modes, Command and Data. You can keep this pin disconnected, and use the slide switch to select the mode. Or, you can control the mode by setting this pin voltage, it will override the switch setting!  High = Command Mode, Low = UART/DATA mode. This pin is level shifted, you can use 3-5V logic
  • DFU: Setting this pin low when you power the device up will force the Bluefruit LE module to enter a special firmware update mode to update the firmware over the air.  Once the device is powered up, this pin can also be used to perform a factory reset.  Wire the pin to GND for >5s until the two LEDs start to blink, then release the pin (set it to 5V or logic high) and a factory reset will be performed.

Reverse Side Breakouts

On the back we also have a few breakouts!

Opt VBat: If you fancy, you can solder on a JST 2-PH connector, this will let you easily plug in a Lithium Ion or other battery pack. This connector pad is diode protected so you can use both Vin and VBat and the regulator will automatically switch to the higher voltage

Opt. 32 KHz: If you're doing some funky low power work, we wanted to give you the option of solderin in a 32khz oscillator. Our firmware doesn't support it yet but its there!

SWC: This is the SWD clock pin, 3v logic - for advanced hackers!

SWD: This is the SWD data pin, 3v logic - for advanced hackers!

3Vo: This is the output from the 3V regulator, for testing and also if you really need regulated 3V, up to 250mA available

FCR: This is the factory reset pin. When all else fails and you did something to really weird out your module, tie this pad to ground while powering up the module and it will factory reset. You should try the DFU reset method first tho (see that tutorial page)


This guide was first published on May 01, 2015. It was last updated on May 01, 2015.

This page (Pinouts) was last updated on Apr 30, 2015.

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