Difference between revisions of "Raspberry Pi"

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(Hardware random number generator: Update to reflect that rng-tools is *not* shipped with ALARM.)
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== Hardware random number generator ==
== Hardware random number generator ==
Arch Linux ARM for the Raspberry Pi is distributed with the ''rng-tools'' package installed and the {{ic|bcm2708-rng}} module set to load  at boot (see [http://archlinuxarm.org/forum/viewtopic.php?f=31&t=4993#p27708 this]), but we must also tell the Hardware RNG Entropy Gatherer Daemon (''rngd'') where to find the hardware random number generator.
Arch Linux ARM for the Raspberry Pi has the {{ic|bcm2708-rng}} module set to load  at boot (see [http://archlinuxarm.org/forum/viewtopic.php?f=31&t=4993#p27708 this]), but you still need to install the {{pkg|rng-tools}} and tell the Hardware RNG Entropy Gatherer Daemon (''rngd'') where to find the hardware random number generator.
This can be done by editing {{ic|/etc/conf.d/rngd}}:
This can be done by editing {{ic|/etc/conf.d/rngd}}:

Revision as of 21:20, 10 April 2015

zh-CN:Raspberry Pi From Wikipedia:

"The Raspberry Pi is a series of credit card-sized single-board computers developed in the UK by the Raspberry Pi Foundation with the intention of promoting the teaching of basic computer science in schools."

The original models, released in 2012, are based on the Broadcom SoC BCM2835 (ARM11 microarchitecture). The Raspberry Pi 2, released in 2015, is shipped with a BCM2836 SoC (quad-core ARM Cortex A7 architecture).

Article Preface

This article is not meant to be an exhaustive setup guide and assumes that the reader has setup an Arch system before. Arch newbies are encouraged to read the Beginners' guide if unsure how to perform standard tasks such as creating users, managing the system, etc.

Note: Support for the ARM architecture is provided on http://archlinuxarm.org not through posts to the official Arch Linux Forum. Any posts related to ARM specific issues will be promptly closed per the Arch Linux Distribution Support ONLY policy.

System Architecture

The Raspberry Pi is an ARM-based device and therefore needs binaries compiled for this architecure. These binaries are provided by the Arch Linux ARM project which ports Arch Linux to ARM-based devices. They also have a separate community and forum on their website, while original forum does not support ARM specific issues. With the introduction of the Raspberry Pi 2 the packages needed now depend on which architecure the devices has:

  • ARMv6 (BCM2835): Raspberry Pi Model A, A+, B, B+
  • ARMv7 (BCM2836): Raspberry Pi 2 (based on Model B+)

Installing Arch Linux ARM

See the Arch Linux ARM Pi documentation or Arch Linux ARM Pi2 documentation.


The fresh install comes with a DHCP client daemon (dhcpcd) preconfigured and will be a connected to a personal LAN provided by almost every router automatically.

If you intend to access your initially prepared SD card installation through Secure Shell the most convenient method is to connect your Raspberry Pi via Ethernet cable and find its assigned IP address through your router configuration. Then you can directly access it as root (password: root). It is highly recommended to change the password and optionally set up SSH keys in the first place.

Configure WLAN without Ethernet

If you need to establish a wireless internet connection, you must download some additional packages and save them to the SD card. If using netctl you need at least WPA supplicant and its dependency libnl. Only for convenience its possible to also install dialog and its dependency ncurses.

So basically you need to

  • find and download the package to the SD card from another computer
  • install and configure packages on the Raspberry Pi

You can find the location of all needed packages on the official website of Arch Linux ARM and download them from their repository.

Subsequently you only need to connect and fire up your Raspberry Pi, log in using monitor/keyboard or the serial console and offline install the packages. If there are any missing dependencies they also must be downloaded and installed

After installing the packages a connection can be established and further configured as told in this section of the Beginner's Guide.


Note: The requisite module snd-bcm2835 should be autoloaded by default.

Install the alsa-utils, alsa-firmware, alsa-lib and alsa-plugins packages:

# pacman -S alsa-utils alsa-firmware alsa-lib alsa-plugins

Optionally adjust the default volume using alsamixer and ensure that the sole source "PCM" is not muted (denoted by MM if muted, press M to unmute).

Select an audio source for output:

$ amixer cset numid=3 x

Where x corresponds to:

  • 0 for Auto
  • 1 for Analog out
  • 3 for HDMI

Caveats for HDMI audio

Some applications require a setting in /boot/config.txt to force audio over HDMI:



HDMI / analog TV-Out

To turn the HDMI or analog TV-Out on or off, have a look at


Use the -s parameter to check the status of your display, the -o parameter to turn your display off and -p parameter to power on HDMI with preferred settings.

Adjustments are likely required to correct proper overscan/underscan and are easily achieved in boot/config.txt in which many tweaks are set. To fix, simply uncomment the corresponding lines and setup per the commented instructions:

# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border

Users wishing to use the analog video out should consult this config file which contains options for non-NTSC outputs.

A reboot is needed for new settings to take effect.

X.org driver

The X.org driver for Raspberry Pi can be installed with the xf86-video-fbdev package:

# pacman -S xf86-video-fbdev

Onboard hardware sensors


Temperatures sensors can be queried with utils in the raspberrypi-firmware-tools package. The RPi offers a sensor on the BCM2835 SoC (CPU/GPU):

$ /opt/vc/bin/vcgencmd measure_temp

Alternatively, simply read from the file system:

$ cat /sys/class/thermal/thermal_zone0/temp

For human readable output:

$ awk '{printf "%3.1f°C\n", $1/1000}' /sys/class/thermal/thermal_zone0/temp


Four different voltages can be monitored via /opt/vc/bin/vcgencmd as well:

$ /opt/vc/bin/vcgencmd measure_volts <id>

Where <id> is:

  • core for core voltage
  • sdram_c for sdram Core voltage
  • sdram_i for sdram I/O voltage
  • sdram_p for sdram PHY voltage

Lightweight Monitoring Suite

monitorixAUR has specific support for the RPi since v3.2.0. Screenshots available here.


The RPi can be overclocked by editing /boot/config.txt, for example:


The optional *_min lines define the minimum frequency to be used for the given component. When the system is not under load, the frequencies will drop down to the minimum value. Consult the Overclocking article on elinux for additional options and examples.

A reboot is needed for new settings to take effect.

The overclocked setting for CPU clock applies only when the governor throttles up the CPU, i.e. under load. To query the current frequency of the CPU:

$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq

See CPU frequency scaling for details on scaling governors.

Tip: The following script will show all frequencies set on the RPi:
for src in arm core h264 isp v3d uart pwm emmc pixel vec hdmi dpi ; do
    echo -e "$src:\t$(/opt/vc/bin/vcgencmd  measure_clock $src)"

Tips for maximizing SD card performance

See Maximizing performance for the general performance improvements.

Enable fsck on boot

Follow fsck#Boot time checking. Remember that kernel parameters are specified in /boot/cmdline.txt.

Serial console

Edit the default /boot/cmdline.txt, change loglevel to 5 to see boot messages:


Change speed from 115200 to 38400:

console=ttyAMA0,38400 kgdboc=ttyAMA0,38400

Start getty service

# systemctl start getty@ttyAMA0

Enable on boot

# systemctl enable getty@ttyAMA0.service

Creating the proper service link:

# ln -s /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/getty.target.wants/serial-getty@ttyAMA0.service

Then connect :)

# screen /dev/ttyUSB0 38400

Raspberry Pi camera module

The commands for the camera module are included as part of the raspberrypi-firmware-tools package - which is installed by default. You can then use:

$ /opt/vc/bin/raspistill
$ /opt/vc/bin/raspivid

You need to append to /boot/config.txt:




If you get the following error:

mmal: mmal_vc_component_enable: failed to enable component: ENOSPC
mmal: camera component couldn't be enabled
mmal: main: Failed to create camera component
mmal: Failed to run camera app. Please check for firmware updates

try setting these values in /boot/config.txt:


If you get the following error:

mmal: mmal_vc_component_create: failed to create component 'vc.ril.camera' (1:ENOMEM)
mmal: mmal_component_create_core: could not create component 'vc.ril.camera' (1)
mmal: Failed to create camera component
mmal: main: Failed to create camera component
mmal: Only 64M of gpu_mem is configured. Try running "sudo raspi-config" and ensure that "memory_split" has a value of 128 or greater

add the following line into the /etc/modprobe.d/blacklist.conf:

blacklist i2c_bcm2708

In order to use standard applications (those that look for /dev/video0) the V4L2 driver must be loaded. This can be done automatically at boot by creating an autoload file such as the following.


Hardware random number generator

Arch Linux ARM for the Raspberry Pi has the bcm2708-rng module set to load at boot (see this), but you still need to install the rng-tools and tell the Hardware RNG Entropy Gatherer Daemon (rngd) where to find the hardware random number generator.

This can be done by editing /etc/conf.d/rngd:

RNGD_OPTS="-o /dev/random -r /dev/hwrng"

and restarting the rngd daemon.

Once completed, this change ensures that data from the hardware random number generator is fed into the kernel's entropy pool at /dev/random.



To be able to use the GPIO pins from Python, you can use the RPi.GPIO library. Install either python-raspberry-gpioAUR or python2-raspberry-gpioAUR from the AUR.


Install i2c-tools and lm_sensors packages.

Configure the i2c-dev module to be loaded at boot:


Reboot the Raspberry Pi and issue the following command to get the hardware address:

 i2cdetect -y 0

Now we need to tell Linux to instantiate the device. Change the hardware address to the address you have found in the previous step with '0x' as prefix (e.g. 0x48) and choose a device name you like:

 echo <devicename> <hardware address> >/sys/class/i2c-adapter/i2c-0/new_device

If you check the kernel messages using the dmesg command, you will see the following new entry:

 i2c-0: new_device: Instantiated device ds1621 at 0x48

Finally, read the sensor output:


QEMU chroot

Sometimes it is easier to work directly on a disk image instead of the real Raspberry Pi. This can be achieved by mounting an SD card containing the RPi root partition and chrooting into it. From the chroot it should be possible to run pacman and install more packages, compile large libraries etc. Since the executables are for the ARM architecture, the translation to x86 needs to be performed by QEMU.

Install binfmt-supportAUR and qemu-user-staticAUR from the AUR.

Make sure that the ARM to x86 translation is active:

# update-binfmts --importdir /var/lib/binfmts/ --import
# update-binfmts --display qemu-arm

Mount the SD card to mnt/ (the device name may be different for you).

# mkdir mnt
# mount /dev/mmcblk0p2 mnt

Copy the QEMU executable, which will handle the translation from ARM, to the SD card root:

# cp /usr/bin/qemu-arm-static mnt/usr/bin

Finally chroot into the SD card root as described in Change root#Using chroot.

See also