🚀 go-pugleaf

tmh@keks.first.gmd.de (Thomas Hoberg) writes:
>In article <3713@svin02.info.win.tue.nl> debra@wsinis07.info.win.tue.nl (Paul De Bra) writes:
>   The 386 uses a 2x clock, whereas the 486 uses a 1x clock.
>   A 2x clock has the advantage of delivering a perfectly symmetrical
>   1x clock after halving. A 1x clock needs better clock circuitry to
>   make sure it is symmetrical.

>Hmm, I was always under the impression that even the 486 used a 2x
>clock up until the 486DX[2]50, which uses a 1x clock. Anyway I have a
>66MHz oscillator on my 486/33 motherboard.

The '486 and '386 both require a more symmetrical clock than can be
provided by standard crystal oscillators. (They come with 60%/40% duty
cycle normally.)  You can buy 55%/45% ones but these are more expensive,
come from fewer suppliers and have longer lead times.

Hence you will find that '386 machines have an oscillator with a
tighter duty cycle spec at twice the cpu frequency(*) and '486 machines
will often have an ordinary oscillator at twice the cpu frequency which
is divided by two to give the correct duty cycle.  However, with a
'486DX-50, this becomes very difficult.  Particularly if you need to
meet FCC and CISPR RFI standards.

*note: TTL crystal oscillators top out at ~80MHz which is twice the top
frequency that you can buy '386s in.  Funny about that :-)

-- 
Scott Colwell				Labtam Australia Pty. Ltd.
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