Linux driver for the Nordic nRF905 transceiver

Introduction

The driver provides a character device and a few sysfs attributes, allowing for simple wireless communication with another Raspberry Pi or perhaps an Arduino with the same nRF905 chip.

Character device

/dev/nrf905

Allowed operations are write and read. The file can be opened for both read and write operations (r+ mode, see man 3 fopen for further details).

Read and write operations only work with 32 bytes at a time (the size of the buffer on the nRF905 chip). If a different size is used, EMSGSIZE is returned.

Sysfs attributes

/sys/bus/spi/devices/spi0.0/frequency

The frequency (in kHz) that the transceiver is tuned to. Allowed ranges are [422400, 473600) and [844800, 947200), with rounding to 100kHz in the former and 200kHz in the latter case. The lower bound is included in both cases, the upper is excluded. Both HFREQ_PLL and CH_NO are set based on the frequency written to this sysfs attribute.

/sys/bus/spi/devices/spi0.0/pa_pwr

Output power. Allowed values are from 0 to 3 (both inclusive).

• 0: -10dBm
• 1: -2dBm
• 2: +6dBm
• 3: +10dBm

/sys/bus/spi/devices/spi0.0/rx_address

The address used when receiving. At the moment, the driver supports only 4-byte addresses.

/sys/bus/spi/devices/spi0.0/tx_address

The address used when transmitting. At the moment, the driver supports only 4-byte addresses.

Simple usage

General configuration

This needs to be done only once per boot.

sudo sh -c "echo -n ABCD > /sys/bus/spi/devices/spi0.0/rx_address"
sudo sh -c "echo -n ABCD > /sys/bus/spi/devices/spi0.0/tx_address"
sudo sh -c "echo -n 868000 > /sys/bus/spi/devices/spi0.0/frequency"
sudo sh -c "echo -n 3 > /sys/bus/spi/devices/spi0.0/pa_pwr"

Receiver

sudo dd if=/dev/nrf905 bs=32 count=1 2>/dev/null | hexdump -Cv

Transmitter

sudo dd if=/dev/urandom bs=32 count=1 of=/dev/nrf905

For normal usage, you should change the permissions (see man 1 chmod), but that is out of the scope of this document.

Installation instructions

Install Raspbian Jessie

At the moment of writing, the current version is 2016-11-25 and the lite edition has been used while writing this document.

See https://www.raspberrypi.org/documentation/installation/installing-images/README.md for instructions on how to make a bootable SD card.

Update the system

sudo apt-get update
sudo apt-get dist-upgrade

Install a kernel with exported symbols

sudo apt-get install linux-image-4.4.0-1-rpi2 linux-image-rpi2-rpfv linux-headers-rpi2-rpfv

Note: Use rpi instead of rpi2 if you have the original Raspberry Pi.

Set the default kernel

Edit /boot/config.txt and add the following lines

# use the rpfv kernel
kernel=vmlinuz-4.4.0-1-rpi2
initrd=initrd.img-4.4.0-1-rpi2 followkernel

Note: Use rpi instead of rpi2 if you have the original Raspberry Pi.

Boot with the new kernel

sudo reboot

Verify the current kernel

uname -r

If everything is correct, the output should be

4.4.0-1-rpi2

Clone this repository

git clone https://github.com/ICBTech/nRF905.git

Build the module against the currently running kernel

make
make dtb

Install the module and overlay

sudo make modules_install
sudo make dtb_install
sudo depmod -a

Manually load the module

sudo modprobe nrf905

Check that the module has been loaded

dmesg | grep nrf905

If everything is correct, the output should be

[   79.829085] nrf905_init

The timestamp would, of course, be different.

Enable the overlay

Edit /boot/config.txt and add the following lines

# load our custom overlay
dtoverlay=nrf905

In that same file, also uncomment the line to enable spi

# enable spidev
dtparam=spi=on

sudo reboot

Manually load the module

sudo modprobe nrf905

Check that the module has been loaded

dmesg | grep nrf905

If everything is correct, the output should be

[   25.095296] nrf905_init
[   25.096562] nrf905 spi0.0: nrf905_probe
[   25.096615] nrf905 spi0.0: gpio_pwr_up: 17
[   25.096627] nrf905 spi0.0: gpio_trx_ce: 27
[   25.096638] nrf905 spi0.0: gpio_tx_en: 22
[   25.096649] nrf905 spi0.0: gpio_dr: 18

The timestamps would, of course, be different. Compared to the previous run of the same command, the output now also contains lines where the driver is probed (bound to the spi address defined in the dts file).

Automatically load the module at boot

Edit /etc/modules and add the following line

nrf905