Page 85 - A Practical Guide from Design Planning to Manufacturing

Computer Components 61

If the VR cannot react quickly enough to dips or spikes in the proces-
sor’s current draw, the processor may fail or be permanently damaged.
The large currents and fast switching of the VR transistors cause them
to become yet another source of heat in the system. Limiting the max-
imum current they can supply will reduce VR heat and cost, but this may
limit the performance of the processor.
To reduce average processor and VR power and extend battery life in
portable products, some processors use VID to dynamically vary their
voltage. Because the processor controls its own voltage through the VID
signals to the VR, it can reduce its voltage to save power. A lower volt-
age requires running at a lower frequency, so this would typically only
be done when the system determines that maximum performance is
not currently required. If the processor workload increases, the voltage
and frequency are increased back to their maximum levels. This is the
®
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mechanism behind Transmeta’s LongRun , AMD’s PowerNow! , and
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Intel’s Enhanced SpeedStep technologies.
A small battery on the motherboard supplies power to a Real Time
Clock (RTC) counter that keeps track of the passage of time when the
system is powered down. The battery also supplies power to a small
memory called the CMOS RAM that stores system configuration infor-
mation. The name CMOS RAM is left over from systems where the
processor and main memory were made using only NMOS transistors,
and the CMOS RAM was specially made to use NMOS and PMOS,
which allowed it to have extremely low standby power. These days all
the chips on the motherboard are CMOS, but the name CMOS RAM per-
sists. Modern chipsets will often incorporate both the real time clock
counter and CMOS RAM into the Southbridge chip.
To create clock signals to synchronize all the motherboard compo-
nents, a quartz crystal oscillator is used. A small sliver of quartz has a
voltage applied to it that causes it to vibrate and vary the voltage signal
at a specific frequency. The original IBM PC used a crystal with a fre-
quency of 14.318 MHz, and all PC motherboards to this day use a crys-
tal with the same frequency. Multiplying or dividing the frequency of this
one crystal creates almost all the clock signals on all the chips in the com-
puter system. One exception is a separate crystal with a frequency of
32.768 kHz, which is used to drive the RTC. This allows the RTC to count
time independent of the speed of the buses and prevents an overclocked
system from measuring time inaccurately.
The complexity of motherboards and the wide variety of components
they use make it difficult to write software to interact directly with
more than one type of motherboard. To provide a standard software
interface every motherboard provides basic functions through its own
Basic Input Output System (BIOS).