Raspberry Pi A, User Manual, Page: 130 / 314

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P A R T I
C O N N E C T I N G T H E B O A R D
For the changes to take effect, the Pi must be restarted. If you find that your changes have
disabled the Pi’s video output on your monitor, simply insert the SD card into another com-
puter and either modify the
config.txt
file with new settings or delete it altogether to
restore the defaults.
Boot Options
The
config.txt
file can also be used to control how Linux is loaded on the Raspberry Pi.
Although the most common method for controlling the loading of the Linux kernel is to use
a separate file called
cmdline.txt
(which you’ll learn about later in this chapter), it is pos-
sible to use just
config.txt
. The following options control the boot process:
❍
disable_commandline_tags —This tells the
start.elf
module that loads first dur-
ing the Pi’s boot process to skip filling in memory locations past 0x100 before loading
the Linux kernel. This option should not be disabled, as doing so can cause Linux to
load incorrectly and crash.
❍
cmdline
—The command line parameters to be passed to the Linux kernel. This can be
used in place of the
cmdline.txt
file, usually found in the
/boot
directory.
❍
kernel —The name of the kernel file to be loaded. This can be used as a way to load the
emergency kernel (see Chapter 4, “Troubleshooting”).
❍
ramfsfile
—The name of the initial RAM file system (RAMFS) to be loaded. This should
rarely be modified, unless you’ve built a new initial file system with which to experiment.
❍
init_uart_baud —The speed of the serial console, in bits per second. The default is
115200, but lower values may improve the connection if the Pi is used with an older
hardware serial terminal.
Overclocking the Raspberry Pi
The
config.txt
file not only controls the graphics outputs of the Pi’s BCM2835 processor
but also enables you to control the chip in other ways. In particular, it allows you to alter the
speed at which the chip runs, increasing its performance at the expense of the part’s lifes-
pan—a process known as
overclocking
.
WARNING
Adjusting any of the settings listed in this section can result in damage to your Pi. In particular,
changing settings corresponding to memory, GPU or CPU voltages will set a fuse in the chip,
which invalidates the Raspberry Pi’s warranty even if the setting is returned to normal before any
damage is done. Damage caused when using these settings will not be put right by the Raspberry
Pi Foundation or by the retailer from whom you purchased your Pi. If in doubt,
don’t alter these
settings : the performance gains through overclocking are rarely worth the risk to the Pi.