NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/sci/electron/20810

Internet Message Format | 1992-12-15 | 3.7 KB

Path: sparky!uunet!olivea!spool.mu.edu!hri.com!ukma!eng.ufl.edu!omaha.eel.ufl.edu!iqbal
From: [e-mail] (M.Iqbal)
Newsgroups: sci.electronics
Subject: Re: 486DX2 Crystals
Message-ID: <[e-mail]>
Date: 16 Dec 92 01:19:18 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail] (Usenet Diskhog System)
Organization: University of Florida
Lines: 63

In article <[e-mail]>, [e-mail] (Kees van der Wal) writes:
|> In article <[e-mail]>, [e-mail] (Mr
|> L A J Davis) writes:
|>
|> > I purchased a 486 motherboard fitted with a 486DX2 50MHz cpu. Unfortunately
|> > with no extra cooling the system fails to complete its bootup proceedure.
|> > The board is fitted with 33.3MHz crystal that can be replaced presumably
|> > for the different 486 cpu's the board is able to support.
|> > By cooling the cpu with a cold can out the fridge(very scientific) and
|> > I have managed to briefly run norton si which claims my processor is
|> > a 67MHz 486. My guess is that the crystal should be 25MHZ for the cpu
|> > to operate however on contacting the supplier he claimed that the crystal
|> > frequency didnt matter so long as it is high enough and reckons i need a
|> > 50 or 80 Mhz xtal which he is posting to me. Is this assumption correct?
|>
|> My Taiwan motherboard with 486DX2-66 has a crystal of 33.33MHz. The
|> documentation prescribes a 25MHz crystal for a 50Mhz DX2 and a 50Mhz version
|> for a 50MHz DX. Also some jumpers have to be set.
|>
|> > Logically I would say that the cpu frequency is directly related to the
|> > xtal frequency and therefore, as I have a dx2 50 the xtal should be 25MHz
|> > (or possibly some multiple of that)
|>
|> This makes sense to me. Though I still wonder where my DX2 get's its 66MHz
|> clock from if the crystal runs at 33MHz only. Does it contain some internal
|> clock doubler circuitry?

Square wave have infinte number of frequencies: a fact explained in
any introductory communication book. If you do not believe me than
try finding Fourier Transform of a pulse train or see a specturm
of a pulse train on a spectrum analyzer.

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.

There are all sort of issues involve in using 2nd harmonic like the
energy of the second harmonic quite low. I have do not know what
actually microprocessor manufactures work around this problem. It may
be possible that they use an amplifier, but than one has to sacrify
chip area for speed or user shorter pulses, thus, giving more energy
in the second harmonic.

I hope this would have helped.


--

---------------------------------------------------------------------------
"If your annual income today is $50,000, you have the same buying power as
the average coal miner did in 1949, adjusted for taxes and inflation," John
Sestina, nationally recognized Certified Financial Planner;  quoted in 1987.

                               >><<<     >>><<<<
[e-mail]                  (o o)     (o) (o)
University of Florida                 [ | ]        |
                                             -         ---