NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/electron/20879

Internet Message Format | 1992-12-16 | 1.8 KB

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From: [e-mail] (Sehat Sutardja)
Newsgroups: sci.electronics
Subject: Re: 486DX2 Crystals
Message-ID: <[e-mail]>
Date: 16 Dec 92 19:47:06 GMT
References: <[e-mail]>
Organization: Integrated Information Tech.
Lines: 28

From article <[e-mail]>, by [e-mail] (M.Iqbal):


> Anyways, one should be able see that square wave have harmonics all over
> the frequency band. What CPU is doing is picking up a harmonic of the input
> clock frequecny. Therefore, second harmonic of 25 mhz is 50 mhz and similarly
> second harmonic of 33 mhz is 66 mhz. Usually, a CPU has a Phase lock loop
> to lock on the second harmonic of the clock frequency; thus, running
> the internal circuitry of the porcessor at 50 or 66 mhz. One can, however,
> pick up second harmonic by using a filter, but, it is much easier to build
> a Phase lock loop on a silicon wafer than passive components like R, L,
> and C.
>

Actually, A CPU PLL normally does not pick up any harmonic component of the
input signal. In fact, the way the CPU PLL works is to compare one edge (for
example rising edge) of the input clock to the edges of the internal oscillator
circuit. In order to get 2x internal clock, the internal oscillator is divided
by 2 before the edge comparison. The frequency of the internal osc is then
adjusted such that for every input edge there is corresponding edge of the
divided internal osc (a phase-freq detector is normally used to do this).
If you need to find more information on the circuit used in the 486DX2, pick
up a copy of the latest JSSCC.
--
Sehat Sutardja,

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