Overclocking may have only become a mainstream hobby
is the last decade, but its roots can be traced back to the
PC itself. While the methods used to overclock may have
changed, the goal has remained the same – “increase the
performance of your computer”. Much like modifying a car's
engine, overclocking can be done for one or two reasons.
Either you can increase the performance of a cheap model to
match that of a more expensive model, aim for performance
greater than that of any off-the-shelf components or both.
HOW IT WORKS
A clock speed is generated by a clock
crystal known as a crystal oscillator,
Quartz usually the crustal of choice.
When an electric current is passed
through the crystal, it produces a
steady pulse of 14.318MHz - this
is called the system clock. It is not
unique to computers and can be seen
in televisions amongst other things.
However, to the computer this is a
seemingly useless frequency as
nothing runs at 14.318MHz.
To combat this problem, a circuit is
used to derive the speeds required.
This circuit is known as a PhaseLocked Loop, or PLL. Several of these
circuits can be seen in a single clock
generator, which is a chip running
alongside a nearby 14.318MHz
oscillator. The clock generator gives
off the required frequencies for the
BCLK, PCI-Express bus (if it is not
tied to the BCLK), PCI bus, RAM and
anything else which needs a clock
speed.
The PLL allows us to change these
speeds through a few components
- a phase comparator, a Voltage
Controlled Oscillator (VCO), and a
down counter (divider). The phase
comparator has two inputs; one
for the reference clock speed of
14.318MHz and the other for is a
looped-back input for the frequency
from the VCO. The phase comparator
then compares the two frequencies
and generates a voltage based on the
difference. Should both values be the
same, the output voltage will remain
unchanged and the VCO will continue
to oscillate at the current rate of
14.318MHz - the same rate as the
crystal oscillator.
In order to generate a faster clock
speed, a divider is introduced after
the VCO as it is looping back to the
phase comparator. If a divider of two
were to be used, the VCO would feed
in 14.318MHz to the divider which
would result in an output of 7.159MHz.
This 7.159MHz frequency is then
fed back into the phase comparator
which tricks it into thinking it is
only operating at half the frequency
it should. The phase comparator
compensates for this by hitting the
VCO with more voltage to get the
output frequency doubled back up to
14.318MHz.
The result of this trickery is an
actual output of 28.636MHz which is
siphoned off and becomes the new
output frequency for the system. The
divider feeds 14.318MHz back to the
phase comparator, which assumes
that the previous voltage output was
correct. It then continues to output
the same voltage, keeping the higher
system speed.
The IBM XT computer ran at a clock
speed of 4.77MHz. We can see that
reversing this process by introducing
a multiplier into the system instead
of a divider would result in a lower
system speed. By using a multiplier
of three, the phase comparator would
be fed an input of 42.954MHz from the
VCO. It compensates for this, reducing
the voltage until the VCO outputs just
4.773MHz. This 4.773MHz is once
again fed through the multiplier
which in turn reports the current
frequency to the phase comparator as
14.318MHz.
Issue 24 | 2013 The OverClocker 21