Nordic nRF52

The nRF52 series of chips from Nordic Semiconductor are based around an ARM Cortex-M4 core running at 64 MHz and feature Bluetooth Low Energy (BLE) support.

Memory Map

nRF52832

Block Name	Start Address	Length
FLASH	0x00000000	512K
RAM	0x20000000	64K

nRF52840

Block Name	Start Address	Length
FLASH	0x00000000	1024K
RAM	0x20000000	256K

Clock Configuration

Clock settings are handled via Kconfig options, which determines whether to start external crystal for the HFCLK, whether to start the LFCLK and which oscillator to use.

System Timer

The clock used for providing system time can be chosen via Kconfig. You can choose to use ARM SysTick or use RTC in tickless mode.

Regulator Control

DC/DC regulator can be made to be enabled at boot via Kconfig.

Peripheral Support

The following list indicates peripherals supported in NuttX:

Peripheral	Support	Notes
GPIO	Yes
GPIOTE	Yes
I2S	No
MWU	No
NFCT	No
PDM	No
PPI	Yes
PWM	Yes
QDEC	Yes	Quadrature encoder
QSPI	Yes
RADIO	Yes	BLE, IEEE 802.15.4
RNG	Yes
RTC	Yes
SAADC	Yes
SPIM	Yes
SPIS	No
TEMP	No
TIMER	Yes
TWIM	Yes
TWIS	No
UART	Yes
UARTE	No
USBD	Yes
WDT	Yes

Peripherals such as AAR, ACL, CCM, ECB are not directly used by NuttX since they are part of BLE controller implementation (link).

GPIO/GPIOTE

Pins can be configured/operated using nrf52_gpio_* functions. Interrupts are handled via the GPIOTE peripheral in one of two ways: via a GPIOTE channel or via PORT events. The former allows for simultaneous rising/falling edge-sensitive interrupts per-pin. However, as there are a limited number of channels (and sometimes these are used by some drivers for specific tasks), it may not always be possible to use this mechanism. The latter approach for pin interrupts is via the PORT event, determined by pin state on a their corresponding GPIO port. This is related to the SENSE capability of pins, which can only be set to either rising or falling edge sensing.

Depending on CONFIG_NRF52_PER_PIN_INTERRUPTS option, you can set a callback for the PORT event itself or you can set a callback for a given pin. In the latter case the driver scans for pins with DETECT bit high and calls the configured callback automatically.

Finally, GPIOTE can also be used to configure a channel in task mode, which allows to control pin state via tasks/events.

ADC

The SAADC peripheral is exposed via standard ADC driver. The lower-half of this driver is initialized by calling nrf52_adcinitialize().

Three sampling trigger modes are available:

CONFIG_NRF52_SAADC_TASK: each ANIOC_TRIGGER starts one scan of all configured channels.
CONFIG_NRF52_SAADC_TIMER: the internal SAADC timer supports one channel only. The sample rate is 16000000 / CONFIG_NRF52_SAADC_TIMER_CC Hz, with a compare value from 80 to 2047 (about 7.8 kHz to 200 kHz).
CONFIG_NRF52_SAADC_TIMER_PPI: a general-purpose TIMER compare event triggers a scan over PPI. This supports up to eight channels and lower scan rates through the TIMER prescaler.

For TIMER/PPI sampling, enable the TIMER instance selected by CONFIG_NRF52_SAADC_PPI_TIMER and reserve it for the ADC. Select an unused PPI channel with CONFIG_NRF52_SAADC_PPI_CHANNEL. The scan rate is 16000000 / (2^CONFIG_NRF52_SAADC_PPI_PRE * CONFIG_NRF52_SAADC_PPI_CC) Hz. For example, these settings select TIMER2 and a 1 kHz scan rate:

CONFIG_NRF52_SAADC=y
CONFIG_NRF52_TIMER2=y
CONFIG_NRF52_SAADC_TIMER_PPI=y
CONFIG_NRF52_SAADC_PPI_TIMER=2
CONFIG_NRF52_SAADC_PPI_CHANNEL=0
CONFIG_NRF52_SAADC_PPI_PRE=4
CONFIG_NRF52_SAADC_PPI_CC=1000
CONFIG_NRF52_SAADC_CHANNELS=2

The board must pass the channel configuration to nrf52_adcinitialize; CONFIG_NRF52_SAADC_CHANNELS sets the maximum number of channels. With either timer source, issue ANIOC_TRIGGER once to start sampling.

CONFIG_NRF52_SAADC_CONTINUOUS enables double-buffered EasyDMA with either timer source. A second PPI channel restarts the SAADC on each END event. For example, add:

CONFIG_NRF52_SAADC_CONTINUOUS=y
CONFIG_NRF52_SAADC_CONTINUOUS_BUFLEN=32
CONFIG_NRF52_SAADC_CONTINUOUS_PPI_CH=1

CONFIG_NRF52_SAADC_CONTINUOUS_BUFLEN counts whole scans per buffer. With two configured channels and a buffer length of 32, each buffer holds 64 samples in channel order: 0, 1, 0, 1, .... Completed buffers are delivered to the ADC upper half with a per-sample channel map. The number of channels multiplied by the buffer length must not exceed 32767. Single-channel buffer sizing is unchanged.

The sampling and restart PPI channels must differ and must be unused by other drivers. Both channel selections are restricted to 0–15 when CONFIG_NRF52_SOFTDEVICE_CONTROLLER is enabled, or 0–19 otherwise.

I2C

I2C is supported both in polling and interrupt mode (via EasyDMA).

Note

The I2C peripheral does not support sending two transfers without sending a START nor RSTART. For this reason, this is supported via an internal buffer where messages will be first copied to and sent together.

The lower-half of I2C bus is initialized by nrf52_i2cbus_initialize(). There’s also a software (bitbang) I2C implementation for nRF52. The lower-half is initialized via nrf52_i2c_bitbang_initialize().

SPI

SPI is supported both in polling and interrupt-based (via EasyDMA) mode. The latter supports arbitrarily long transfers using Nordic’s list-mode EasyDMA (intermediate transfers are currently still manually started).

It is possible to use SPI without either MOSI/MISO pin defined by simply not providing the relevant BOARD_SPI*_MISO/MOSI_PIN definition.

This implementation support power management hooks, which will disable SPI peripheral when entering either SLEEP or STANDBY modes and reconfigure it when going back to NORMAL mode.

UART

UART is implemented using polling. UARTE EasyDMA feature is not yet supported. This may introduce a large number of interrupts which may be undesirable.

PPI

The PPI peripheral is supported via a specific API which lets you control the EVENT and TASKs to trigger, both for individual and grouped channels.

When using channels, you should consider that some peripherals may use PPI internally and some may be unavailable for further use. As a helper, if debug assertions are enabled, calls to PPI API will check for a channel to actually be disabled when being enabled and vice-versa. This may help catch collisions in PPI use.

PWM

PWM is supported via standard driver. This means that more advanced features such as complex sequences or waveform modes are not yet supported.

QDEC

The QDEC (Quadrature Decoder) peripheral is supported via the standard qencoder driver interface. The LED output is not supported now.

QSPI

QSPI is supported both in interrupt-based (via EasyDMA) mode and is exposed via standard QSPI interface.

RNG

The RNG peripheral will be used to register a random/urandom device automatically, when enabled.

TIMER

The TIMER peripheral is exposed as standard timer.

RTC

The RTC peripheral is exposed as a standard timer, since it is really a low-power timer, without any date handling capabilities.

USBD

The USBD peripheral is exposed via standard USBDEV interface.

WDT

The watchdog is supported via low-level API interface and also via standard watchdog driver. The driver is written so as to handle an already running watchdog, which may have been set by a bootloader.

BLE Support

BLE is supported in nRF52 using Nordic’s SoftDevice Controller, using HCI interface. To enable BLE support you need to call nrf52_sdc_initialize() on boot, which will initialize the BLE controller.

SDC support involves registering various high-priority zero-latency interrupts and thus requires enabling BASEPRI and high-priority interrupt support. On supported boards, a sample sdc configuration is provided with settings already set.

Note that in this case, some peripherals (mostly those related to BLE) will be unavailable. Some PPI channels will also be ocuppied (NRF52_PPI_NUM_CONFIGURABLE_CHANNELS will be set accordingly in this case).

IEEE 802.15.4 Support

Details about IEEE 802.15.4 support for nRF52 can be found in Nordic nRF52 IEEE 802.15.4.