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Path: utzoo!linus!philabs!micomvax!othervax!ray
From: ray@othervax.UUCP (Raymond D. Dunn)
Newsgroups: net.micro.pc
Subject: NEC V20 Caution
Message-ID: <710@othervax.UUCP>
Date: Wed, 9-Oct-85 15:02:54 EDT
Article-I.D.: othervax.710
Posted: Wed Oct  9 15:02:54 1985
Date-Received: Sun, 13-Oct-85 03:16:42 EDT
Reply-To: ray@othervax.UUCP (Raymond D. Dunn)
Organization: Philips Information Systems - St. Laurent  P.Q., Canada
Lines: 31
Keywords: NEC,V20,8088
Summary: V20 timing requirement differ from 8088


Although many, including myself, are successfully using an NEC V20 as a
direct replacement for the Intel 8088 in our PC's, it looks as if this may
not be a good idea in all but a hacking environment.

I am indebted to my colleague Humphrey Brown, for the following information:-

"The following caution should be issued in connection with the widely touted
NEC V20 microprocessor (P/N 70108) as a plug-in upgrade replacement for the
8088 in personal computers.

The specification for the V20 calls for a clock with a 50% duty cycle.  The
clock driver for the 8088 delivers a clock of 33% duty cycle.  NEC's data
sheet for the V20 specifies a minimum clock high time of 85nS and a low time
of 90nS.  Intel gives a minimum clock high time for the 8284 of 
1/3 tclcl+2nS, which for a clock frequency of 4.77Mhz, yields 71nS.

While the chances are that this will work anyway, those who wish to try it
should note that there are no guarantees.

I have'nt got the data sheet for the 8Mhz part yet, but by judicious
extrapolation, that part will require a clock high time of about 48nS, thus
that part of the clock requirement will be met.  However, NEC are quite
explicit in their data sheets as to the need for a 50% duty cycle.

The solution to the mismatch between the Intel clock generator and the NEC
processor might be to also plug in the NEC clock driver (P/N 71011) in place
of the 8284, since it appears to be pin-compatible, but note that the 71011
divides the external frequency by 2, not 3 as for the 8284."

Ray Dunn.   ..philabs!micomvax!othervax!ray