On Thu, 9 Jul 1998, Scott Walsh wrote:

>      A few bods have mentioned that they are annoyed by the way in which a
>      large number of micros divide down the external clock, resonator or
>      crystal usually, and run instructions at that speed ....
>
>      What are the reasons for people finding this so distasteful? I
>      understand that clock division and PLL circuits can be noisey, but I
>      get the feeling that this is not the only reason.
>
>      regards,
>      SW.

The reason is, that manufacturers do not advertise MIPS, they advertise
clock frequency, so, for example, the original 8051 looked 'smashing' with
12 MHz while it was way under 1 MIPS.

It is not good for engineers to have to wade through 1/2 pound of
press releases and advertising comparisons to find out that each cycle of
that fantastic-looking 25 MHz processor takes 11 clocks, so it's really a
2.0-2.2 MIPS processor.

Next, high frequency sources contribute to power drain and to price. The
cost of a 25 MHz crystal is higher than that of a 4 MHz one. It is also
harder to make a stable oscillator at higher frequencies in a production
environment.

Nearly all systems that use a clock over 30 MHz use a 'canned' oscillator
or specially ordered resonators because of this. A replaced ceramic load
capacitor type (same spec, different supplier)  because of part supply
problems and-hop-no oscillation in 300 units or so, until someone notices
at QC and stops the pipeline. Both canned oscillators and special
resonators are expensive, and canned osciallators draw more power than a
dozen CMOS micros in parallel.

This kind of things never seem to happen under 12 MHz and above 1 MHz,
which is the range of 'best stability' crystals for the present technology
(at least it looks like that).

So, people who work with micros and have to make choices HATE this kind of
publicity and the kind of explaining they have to do to the boss and to
the client when not choosing the 'fastest' part (bosses tend not to read
about the gory small details in pamphlets ;).

In micros, MIPS/MHz and MIPS/mA count more than other considerations,
especially in high performance portable equipment. It's not like an
industrial PC where you just upgrade to 333 MHz (at a power drain of 9
Amps for the CPU alone !) only to have a CPU load of 0.1 90% of the time.

Peter