Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!mark
From: mark@mips.com (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: What's a shmoo plot?
Message-ID: <1652@spim.mips.COM>
Date: 30 Mar 91 20:32:30 GMT
References: <7905@uceng.UC.EDU>
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In article <7905@uceng.UC.EDU> lbechtle@uceng.UC.EDU (laurie bechtler) writes:
  >a shmoo plot is just a graphical representation of some kind
  >of circuit condition versus two input variables.  Pick
  >an output criterion, say, does the circuit pass some specific functional
  >test?  Place an X everywhere on the plot that the circuit passes the
  >test.  You end up with some funny blob shape full of X's that shows
  >you where (on that particular set of axes) the circuit functions.


And the classic shmoo (often printed as "schmoo") plot is supply voltage
on the X axis, access time (or clock frequency) on the Y axis.  For example,


  ACCESS TIME                         "X" = PASS
     (ns) 
     15.0 + XXX
     14.0 | XXXX
     13.0 | XXXX
     12.0 | XXXXXX
     11.0 | XXXXXXXX
     10.0 + XXXXXXXXXXXX
      9.0 | XXXXXXXXXXXXXXXXXXX
      8.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      7.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      6.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      5.0 + XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      4.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      3.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      2.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      1.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      0.0 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
     -------+--------+--------+--------+--------+--------+--------+--  VDD
           3.0v     3.5v     4.0v     4.5v     5.0v     5.5v     6.0v  (volts)


This device (a memory chip) slows down dramatically as the supply voltage
is reduced.  At 5.5v it's a 6.0 nanosecond chip, but at 4.5v it's 8ns.

"Shmoo" is derived from a character in the Lil Abner comic strip and
not from "mho" the units of inverse resistance (conductance).
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086
	(408) 524-8308    mark@mips.com  {or ...!decwrl!mips!mark}