It's also down to production the run and the cost of testing, the dies for
the chip are on a disk of silicon about 6" dia?... the center is in focus
for the photo etching process of making the sinks and lands ect. the outer
chips are out of spec.. because the etched interconnections of the chip are
not uniform or out of focus. So they are not tested as much as the good one
from the center of the silicon plater or plate. the yeild of good chips are
taken from the center and the outers are downgraded as a lower speed device.
also the impurities are less from the center.

Art
----- Original Message -----
From: Brandon, Tom <Tom@PSY.UNSW.EDU.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, March 22, 2000 11:37 PM
Subject: Re: Rated PIC speed ?


| Don't know about PICs, but PC CPU's are a similar idea I believe. The
basic
| principle is that 1 atom of contaminant in the silicon noticably effects
the
| heat conductance of the chip at that point. You can't stop single atoms of
| impurities forming only cut them down, we just don't have the needed
| technology. So, Intel (or whoever) has one production line for say 3 diff.
| chips, 200, 233, 266. All the same die as Mark pointed out, the only
| difference is the amount of impurity.
|
| So a lower speed graded chip just has a little more impurity meaning it
| heats up a little more. It's actually a large difference in heat
dissipation
| at the scale they work on but overall it's minimal. i.e. a slower chip
won't
| neccesarily run hotter at a macromolecular scale just in the silicon there
| will be hot spots.
|
| NB: I'm pretty sure of this but don't count on it.
|
| Tom.
|
| -----Original Message-----
| From: Mark Willis [mailto:mwillis@FOXINTERNET.NET]
| Sent: Thursday, March 23, 2000 8:58 AM
| To: PICLIST@MITVMA.MIT.EDU
| Subject: Re: Rated PIC speed ?
|
|
| My understanding is that when the needed quantities of higher speed
| rated testing chips are done, the rest of the same identical parts are
| tested at the lower clock speed and then sold as the slower part.  Most
| all probably *would* test OK at the higher speeds.  All are identically
| constructed on the same equipment, no electrical or physical differences
| (the occasional die shrink happens occasionally, but that happens to ALL
| the parts, not just the higher speed ones <G>)  They may test at slower
| speed first, know how to look it up so I don't store that info <G>
|
| <SNIP>
|
|   Mark
|
| Tobie Horswill wrote:
| > Hi,
| >
| >     I've just noted that the last batch of PICs I bought (16f84 and
| 16f877)
| > were of the 04/P type. I've been running the F84 at 8 Mhz and the F877
at
| > 20Mhz without any problems, they don't seem to be overheating.
| >
| >     What's the physical difference between de 4Mhz and the 10 or 20Mhz
| > versions and do I risk damaging them if they are run overclocked for
long
| > periods ?
| <SNIP>
| >
| > Thanks,
| >
| > Tobie
|
| --
| I re-ship for small US & overseas businesses, world-wide.
| (For private individuals at cost; ask.)
|