Raspberry Pi A, User Manual, Page: 132 / 314

116
P A R T I
C O N N E C T I N G T H E B O A R D
•
isp_freq —Sets the clock frequency of the image sensor pipeline, for improving
the capture rate of connected video hardware (such as a camera). The default
speed is 250 MHz.
•
v3d_freq —Sets the clock frequency of the GPU’s 3D rendering hardware, for a
boost in visualisation and gaming performance. The default speed is 250 MHz.
❍
sdram_freq —Sets the clock speed of the random access memory (RAM) chip on the Pi,
to give the entire system a small increase in performance. The default speed is 400 MHz.
❍
init_uart_clock
—Sets the default clock speed of the Universal Asynchronous Receiver/
Transmitter (UART) , used to control the serial console. The default is 3000000, which
sets a speed of 3 MHz. Altering this is likely to have little effect beyond corrupting the
output of the serial console.
❍
init_emmc_clock —Sets the default clock speed of the SD card controller. The default
is 80000000, which sets a speed of 80 MHz. Increasing this value can result in faster
reading and writing from the SD card, but can also lead to data corruption.
As an example, to overclock the CPU to 800 MHz, the GPU to 280 MHz and the RAM to 420
MHz, enter the following options into
config.txt
, one per line:
arm_freq=800
gpu_freq=280
sdram_freq=420
As with adjusting the display configuration, any changes made regarding overclocking won’t
take effect until the Pi is restarted. To return the settings to normal, you can either delete the
entire
config.txt
file or—if you’re using it to control the display settings as well—simply
delete the lines that deal with overclocking, and then restart the Pi.
If you have overclocked your Pi and it no longer boots, either place the SD card into another
computer to edit the configuration and then try again, or hold down the Shift key while the
Pi boots to temporarily disable your new settings and run the Pi at its normal clock speed.
Overvoltage Settings
If you’re overclocking your Pi, you will eventually hit a brick wall past which the device won’t
go. The precise point at which the Pi won’t reliably overclock depends on the individual
device, due to natural variations in the chip introduced during the manufacturing stage. For
some users, this limit may be as low as 800 MHz; others may find that they can push their Pi
as high as 1 GHz (1,000 MHz) without issue.