ard at p850ug1.demon.co.uk
Wed Oct 19 19:36:39 CDT 2005
> > Well sure, why not? If you had an all static-CMOS MPU, you could even take
> > one of those big rheostats used for stage lighting and hook it up to a VFO
> > to provide a variable clock to your CPU. "Captain, if I turn it up any
> > higher, it'll blow for sure!".
> A PDP that'll go all the way up to 11? :-)
Actually, the PDP11/45 has 2 clock circuits. One is the crystal clock
(33MHz) for normal running. The other is an RC clock, with a preset pot
to set the frequency. You can select the latter by sticking a KM11
maintenance board in and flipping the appropriate swtich on said board.
The RC clock will go faster than 33MHz, the idea being that you ensure
that the machine will run at the higher speed, if so, then it will
certainly work at 33MHz.
The HP9825 (and the 9831) use a dual VCO chip for the CPU clock. One of
the osciallators runs at fixed frequency, the output of that is turned
into an approximately triangular wave, and used to freqeuncy-modulate the
other one. The reason, of course, is to spread out the spectrum of the RF
noise coming from the machine. This is mentioned in passing in the
board-swapper service manual, and is fairly clear from the schematics.
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