nRF51 Architecture

nRF51 Architecture

At the heart of the nRF51 architecture are the 32-bit industry standard  ARM Cortex-M0 processor and the 2.4GHz Multi-protocol radio transceiver which supports a range of protocols including Bluetooth low energy, ANT, and proprietary 2.4GHz RF protocols. The popular low-power ARM Cortex-M0 CPU has a very fast start-up time (around 2.5us) and  a fast duty cycling which leads to  a boosted Bluetooth device responsiveness.

nRF51 Block Diagram – Brief

nRF51 Block Diagram – Detailed

Available system peripherals and Digital and Analog I/O (in order as they appear in the detailed block diagram) :

  • 31x GPIO.
  • Hardware Random Number Generator (RNG).
  • 2×24-bit Real Time Clock (RTC).
  • 2×16-bit timers , 1×32-bit timer.
  • Built-in Temp sensor.
  • 128-bit AES ECB/CCM/AAR co-processor( AES Hardware Encryption Accelerator).
  • 2x TWI (2-wire(I2C compatible ) ).
  • UART.
  • 2x Master SPI.
  • Slave SPI.
  • Quadrature Demodulator.
  • 10-bit ADC.
  • GPIOTE( GPIO Task and Events ).
  • 16-channel Programmable Peripheral Interconnect (PPI).
  • Watchdog timer.

The nRF51 series chips supports the following types of  external oscillators that should be present on the hosting PCB or the development board:

High : 16 Mhz (Crystal or RC)
Low: 32 Khz (Crystal, RC or synthesized clock) .  The low frequency is generally used for higher accuracy and lower average power consumption.

Example: the nRF51 Development Kit (DK)  which host the nRF51422 SoC has two crystal oscillator the default one is the 16Mhz  which is the one connected to the SoC , and there is also a 32Khz low frequency crystal oscillator that can be selected.


nRF1-DK schematic

nRF51 Development Kit (DK) – Available oscillators highlighted


There are three chips options in the nRF51 series :

nRF51 Series

Hardware peripherals wise, all these chips have the same hardware peripherals see comparison of hardware and memory . However, when it comes to the supported protocols (Bluetooth low energy, ANT , or propitiatory 2.4Ghz protocols) the nRF51422 supports ANT and Bluetooth low energy protocols . However, the nRF51822 and the nRF51824 supports Bluetooth low energy and 2.4GHz propitiatory protocols. nRF51 series does not support Bluetooth 5.

Common hardware and software architecture among the different chips and consistent peripheral addressing making these chips easy to work with.  The core registers and registers for tasks, events, shorts and interrupts have equal offsets for all peripherals.

Advanced features:

  1. PPI ( Programmable Peripheral Interconnect ) : It  allows different peripherals to interact with each other independent of the CPU . This saves power by minimizing processor active time and at the same time relaxes real time requirements for the processor.

Programmable Peripheral Interconnect Bus

Peripherals using PPI while CPU is asleep

Tasks and Events registers in the peripherals allow to automate tasks among peripherals with minimum CPU intervention

Tasks and Events registers in the peripherals allow to automate tasks among peripherals with minimum CPU intervention


2.Flexible GPIO :  It allows the digital interfaces of the peripherals to be mapped to most pins available on the SoC. This feature is achieved using an embedded pin crossbar . This has a huge advantage in reducing the complexity of the PCB of the device or the development board.

The problem with fixed pins interfaces

Pin crossbar simplifies PCB routing

Pin crossbar allows interface mapping to most pins

Flexible GPIO leads to reduced PCB complexity


3.Automated power management

The purpose of this unit is to minimize active modules and minimize peak and average current peripherals only use power when they need power.

Automated power management. Only needed modules are powered and clocked

4.Highly efficient EasyDMA radio interface with flexible RAM mapped FIFOs.

nRF51 Development Boards:

  • nRF51-DK (nRF51422) : The nRF51 DK is a single-board development board  that has the following key features:
    • Hosts the nRF51422 SoC
    • All GPIO and interfaces available at edge connectors
    • Buttons and LEDs for user interaction
    • I/O interface for Arduino form factor plug-in modules
    • SEGGER J-Link OB Debugger with debug out functionality
    • Virtual COM Port interface via UART
    • Drag and drop Mass Storage Device (MSD) programming
    • mbed enabled
    • Accepts power through:

      • USB
      • External source (1.8V-3.6V)
      • Single 2032 coin-cell battery, onboard battery holder

    The board ID of the nRF51-DK is PCA10028 .

    nRF51-DK Board (PCA10028)

    nRF51-DK Development Kit


  • nRF51-Dongle (nRF51422)  : Key features of this dongle
    ->Hosts the nRF51422 S0C
    ->RGB LED
    ->6 solder pads for GPIO/interface connections
    ->Supports nRFSniffer – Bluetooth Smart protocol sniffing firmware
    ->Supports Master Emulator – Bluetooth Smart Peer connection firmware
    ->All I/O and interfaces available via connectors
    ->USB drag and drop programming and USB Virtual COM port for serial terminal
    ->Segger J-Link and CMSIS-DAP interface for programming and debugging from offline tools and pyOCD
    ->Connector for RF measurements
    ->Pins for power consumption measurements
    ->Accepts power through:

    • USB
    • External source (1.8V-3.6V)
    • Single 2032 coin-cell battery with additional battery holder

    The Board ID for the nRF51-Dongle is PCA10031

    nRF51 Dongle

  • nRF51822 Development Kit (obsolete)


References and pictures:
Nordic Semiconductor