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From: dhinds@portia.Stanford.EDU (David Hinds)
Newsgroups: comp.sys.ibm.pc
Subject: Re: DRAM speed
Message-ID: <1990May29.013625.18990@portia.Stanford.EDU>
Date: 29 May 90 01:36:25 GMT
Article-I.D.: portia.1990May29.013625.18990
References: <2660AECB.29143@paris.ics.uci.edu> <7613@canterbury.ac.nz>
Organization: AIR, Stanford Universit
Lines: 23

In article <7613@canterbury.ac.nz> PHYS169@canterbury.ac.nz (Mark Aitchison, U of Canty; Physics) writes:
>In article <2660AECB.29143@paris.ics.uci.edu>, levine@ics.uci.edu (David Levine) writes:
>> I heard that it is not a good idea to use DRAM that is rated much faster
>> than required, e.g., 70 ns in place of 120 ns.  Ignoring the price
>> difference, is this true?  If so, why?
>> 
>As I understand it, there are two reasons:
>(a) there might be "junk" on the bus for a short time that is not a problem to
>    the slow memory chips, but could be to fast chips 
>(b) faster chips usually consume more power, introduce current pulses on the
>    power supply lines that are more synchronised, etc.

    These things should only apply if the two different speeds of DRAM's
have been made from different masks.  I think I read on the net that all
the normal varieties are made from the same mask regardless of speed, and
are sorted by speed after testing.  So there would be no difference between
a 70ns chip driven at 120ns, and a "native" 120ns chip running at that speed.
When a chip needs to be redesigned to reach its rated speed, then these
compatibility issues would become important.  I think that even 60ns chips
are still made on the same lines as the slower versions.

 -David Hinds
  dhinds@popserver.stanford.edu