Re: CPU AND peripheral clocks

vivek wrote:

Why are peripheral clocks divided by 2 or 4 or by 8 times of the
CPU clock? Why can't they run at CPU core clock speed? This is
just a general question and not restricted to any processor or
architecture.

Basically, because the clock is too fast. There are two or three
primary effects on clock propagation.

One is simple delay. It takes time for a signal to travel along a
line. That line is longer when the peripheral is external to the
chip.

Another is capacitance. A proximate conductor can add loading, and
disturb the effective period of the clock.

A third is gain and trigger points. An amplifier used to
regenerate the clock on the peripheral will have values for these.
They will also disturb the timing.

The fundamental purpose of the clock is to synchronize things.
Often the idea is that signals start to change when the clock
rises, and get used when the clock falls. With the above, and
other (such as noise) disturbances we no longer know when those
signals are valid. It is already amazing that clocks can be as
fast as they are.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
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