Hardware Reference
In-Depth Information
clock(RTC).Themaincrystalisalways14.31818MHz(itmightbeabbreviatedas14.318
or just 14.3), and the RTC crystal is always 32.768KHz.
Why 14.31818MHz?
The original 1981 vintage IBM PC ran at 4.77MHz, a speed derived by taking a
14.31818MHz crystal and using a divider circuit to divide the frequency by 3 to get
4.77MHz. Many people 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
havehadtodotorunitatthatspeedwaschangethemaincrystalfrom14.318MHzto15MHz
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.
Asatestamenttotheirforesight,allmodernPCsarestillcontrolledbya14.318MHzcrystal!
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 hap-
penswhenyouinstalladifferentprocessor?Howdoesthesystemadjustthebusandother
speedstoaccommodate thenewchip?Theansweristhataspecialchipcalleda
frequency
timing generator
(FTG) or
frequency synthesizer
is used in conjunction with the crystal to
derive the actual speeds of the system.
Figure 3.45
shows a portion of a motherboard with
an FTG chip and a 14.318MHz crystal below it.
Figure 3.45
An ICS 9250 frequency timing generator chip with a 14.318MHz crystal.
