Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!mcsun!ukc!cam-cl!news
From: nbvs@cl.cam.ac.uk (Nicko van Someren)
Newsgroups: comp.sys.acorn
Subject: Re: ARM3 upgrade
Keywords: 30Mhz ARM3
Message-ID: <1991May8.102418.21391@cl.cam.ac.uk>
Date: 8 May 91 10:24:18 GMT
References: <9887@suns6.crosfield.co.uk>
Reply-To: nbvs@cl.cam.ac.uk (Nicko van Someren)
Organization: U of Cambridge Comp Lab, UK
Lines: 56

In article <9887@suns6.crosfield.co.uk> djhr@crosfield.co.uk (dave redman) writes:
>I bought a 30MHz ARM3 and fitted it to an A340..
>
>It worked fine until I switched the ARM3 cache on and then...
>
>The machine would run VERY fast for about 3 mins ( from cold ) and then all
>hell breaks loose with modules dying left right and centre.
>
...
>Anyway I think I traced the problem..
>
>  I bought the Atomwide 4Meg memory upgrade and it has the MEMC1a fitted
>  but it also uses a PEEL18CV8 -35 ( fitted between the CPU and MEMC sockets )
>  Now if my maths serves this device can only run at 28.5 MHz ( max ).

It is very unlikely that this is the problem as the PAL between the ARM and
the MEMC only needs to run at RAM speed which is 8MHz on everything except
540/R260s.

>  I have since modded the ARM3 motherboard to run at 25Mhz and everything works
>  fine, however I WOULD like to go back to 30MHz and I propose to do this by
>  replacing the -35 device with a -15.
...
>I am curious abut something else though.. In my VLSI data book it says the ARM3
>runs at 20Mhz.. does this mean a 30Mhz device is just a 20Mhz device that
>happens to run at 30Mhz or is my Nov. 1990 VLSI data book already out of date ?

Indeed you are right.  30MHz ARM3s are not rated as such by VLSI.  The first
few hundred ARM3s all seemed happy at higher speeds so firms like Watford 
decided that 30MHz was O.K..  Later VLSI changed the fabrication process to
increase the yield and at the same time reduced the top speed.  Since then
Aleph One have stopped rating their parts at 30MHz because a few were found
to be unreliable once they got hot. 

>So the moral of this story..
>
>    IF you buy a Watford Electronics 30Mhz ARM3 it may not work..
>
>to be fair.. Watford DID tell me that in SOME machines the 30Mhz beasty doesnt
>work.

I think that that statement is a bit misleading.  It should read 'Some 30MHz
ARM3 cards will not work at all when hot'.  Unless your machine is suffering
from bad power supply droop (not unknown on early 310s) it is very unlikly that
the rest of your machine will effect the speed of your ARM3.  The reason 
for this is that the high frequency signals are confined to the upgrade card 
and never get on to the main PCB.

If Watford sold you the card as a 30MHz unit, send it back, it clearly is not.

Nicko


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| Nicko van Someren, nbvs@cl.cam.ac.uk, (44) 223 358707 or (44) 860 498903    |
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