Operation Manual

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.
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.
If you want to eke a little more performance out of your Pi, there is a way to potentially boost this upper limit: a process known
as overvoltage or overvolting. The Pi’s BCM2835 system-on-chip processor and the associated memory module usually run at
1.2 volts. It’s possible, although potentially inadvisable, to override this default and force the components to run at higher or
lower voltages. Boosting the voltage means boosting the signal within the chip, making it more likely to hit higher speeds—it also
means that the chip runs hotter, reducing its lifespan dramatically compared to overclocking alone.
Setting any of the voltage options in config.txt causes a fuse within the BCM2835 to trip in a way that cannot be reset. It’s a foolproof way to tell if
someone has been attempting to overclock the chip outside its rated specifications, and renders your warranty null and void—even if the cause of
failure is unrelated to overclocking. If you return a Pi for replacement under warranty and the fuse is tripped, it will not be replaced. Do not attempt
to overvolt a Pi that you cannot afford to replace yourself.
Unlike the previously described settings, which are provided in config.txt as absolute values, the voltage adjustment is carried
out using values relative to the Pi’s stock 1.2 V setting. For each whole number above zero, the voltage is increased by 0.025 V
from stock. For each whole number below zero, the voltage is decreased by 0.025 V from stock.
The voltage adjustment settings have upper and lower limits of 8 and -16, equivalent to 0.2 V above stock voltage or 1.4 V
absolute and 0.4 V below stock voltage or 0.8 V absolute. The voltage must be adjusted in whole numbers, and it cannot be
adjusted below 0.8 V (-16) or above 1.4 V (8.)
The following settings are accessible from config.txt:
• over_voltage—Adjusts the BCM2835’s core voltage. Values are given as a whole number (an integer) corresponding to
0.025 V above or below the default value (0,) with a lower limit of -16 and an upper limit of 8.
• over_voltage_sdram—Adjusts the voltage given to the memory chip on the Pi. As with over_voltage, values are given as
a whole number corresponding to 0.025 V above or below the stock (0,) with a lower limit of -16 and an upper limit of 8.
Additionally, you can adjust voltages for individual memory components using the following options:
• over_voltage_sdram_c—Adjusts the voltage given to the memory controller. Acceptable values are the same as with
over_voltage_sdram.
• over_voltage_sdram_i—Adjusts the voltage given to the memory’s input/output (I/O) system. Acceptable values are the
same as with over_voltage_sdram.
• over_voltage_sdram_p—Adjusts the voltage given to the memory’s physical layer (PHY) components. Acceptable
values are the same as with over_voltage_sdram.
As an example, the following lines entered into config.txt will give the BCM2835 a small boost of 0.05 V to 1.25 V and the
memory chip a bigger boost of 0.1 V to 1.3 V:
over_voltage=2
over_voltage_sdram=4
As with other settings, deleting the lines from config.txt or deleting the file itself will return things to normal. Unlike the other
settings in this section, however, the evidence will remain in the form of a blown fuse in the BCM2835—rendering the Pi’s
warranty null and void even after the default settings are restored.
Disabling L2 Cache
The Pi’s BCM2835 SoC processor has 128 KB of Layer 2 cache memory onboard. Although this memory is small, it’s
extremely fast. It’s used to temporarily store—aka cache—data and instructions between the slower main memory and the
processor itself to improve performance.
Because of the BCM2835’s origins as a multimedia processor targeted at set-top boxes, this L2 cache is designed to be used by
the GPU portion of the chip alone. Unlike a traditional processor, the CPU doesn’t have any L2 cache of its own.