Hardware Reference
In-Depth Information
were confused as to why IBM chose to run the processor at 4.77MHz; after all, the 8088
processor they used was rated for 5MHz, and all they would have had to do to run it at that
speed was change the main crystal from 14.318MHz to 15MHz instead. Well, the truth is that if
they did that, they would have had to add more crystals to the design. You see, the same
14.318MHz crystal that was divided by 3 to run the processor was also divided by 4 to get
3.58MHz, which is the
exact
frequency needed for the NTSC color video modulation signal to
be compatible with color TV.
But that's not all: Another circuit divided the crystal frequency by 12 to get 1.193182MHz,
which was used by an 8253 programmable three-channel 16-bit interval timer/counter chip.
Each channel could be used to take an input clock signal and produce an output signal by
dividing by an arbitrary 16-bit number. Channel 0 was used to make the time-of-day clock
ticks. It was programmed by the BIOS to call INT 08h every 65,536 ticks, which was about
18.2 times per second (or about every 55 milliseconds). The software routines linked to INT
08h caused the time-of-day clock to be updated and could chain to any other activities that
needed to be done periodically. Channel 1 was used to tell the DMA to refresh the dynamic
RAM every 72 cycles (about 15 microseconds), and channel 2 was used to make an audio
signal for the speaker (beeps)—different tones could be made by changing the divisor.
So by carefully choosing a 14.318MHz crystal instead of 15MHz or some other speed, the IBM
engineers were able to design a motherboard in which a single crystal could run the processor,
video card, time-of-day clock, memory refresh, and even beep tones. The single-crystal design
allowed the motherboard to be manufactured with fewer parts and at a lower cost.
As a testament to their foresight, all modern PCs are still controlled by a 14.318MHz crystal!
This crystal, in conjunction with a frequency timing generator chip, derives virtually all the
frequencies used on a modern motherboard by the CPU, buses, memory, and more.
PCs don't run at 14.318MHz, so how can a crystal of that speed be used? And what happens when
you install a different processor? How does the system adjust the bus and other speeds to
accommodate the new chip? The answer is that a special chip called a
frequency timing generator
(FTG) or
frequency synthesizer
is used in conjunction with the crystal to derive the actual speeds of
the system.
Figure 3.49
shows a portion of a motherboard with an FTG chip and a 14.318MHz crystal
below it.
