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.
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 |
No |
|
QSPI |
No |
|
RADIO |
Yes |
Basic |
RNG |
Yes |
|
RTC |
Yes |
|
SAADC |
Yes |
|
SPIM |
Yes |
|
SPIS |
No |
|
TEMP |
No |
|
TIMER |
Yes |
|
TWIM |
Yes |
|
TWIS |
No |
|
UART |
Yes |
|
UARTE |
No |
|
USBD |
No |
|
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().
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 undesireable.
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 viceversa. 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.
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.
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).