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'Rated PIC speed ?'
2000\03\22@145344 by Tobie Horswill

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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 ?

   Also, has anyone been able to modify the F877's  SSPSTAT<7,6> bits also
known as SMP and CKE  ? I'm using MPLAB 5.00 and the simulator refuses to
change these bits (banking is ok).

Thanks,

Tobie

2000\03\22@165951 by Mark Willis

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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>

Remember: this testing is done over *all* guaranteed supply voltages and
*all* temperatures, not just the nominal power supply voltage and
temperature you probably use the chip at - So yes, you can overclock.
Do so with care, though.

Works for prototypes and "play projects", I certainly would not do this
for ANY life-critical projects, too much liability for saving a few
dollars;  OTOH, by adding a heat sink, there are reportedly people who
can run a 20MHz part WAY overclocked, and reportedly don't have problems
(in their commercial application, that their livelihood depends on.)
Something like a 20MHz part at 40-50 MHz speeds, which sounds way
overclocked to me.  I haven't tested this;  A Microchip FAE said it, and
I DO believe him! <G>

Moral of the story:  Test your own chips, individually, at the desired
speed and over the expected variety of power supply voltages and
temperatures it'll run at, if you're going to do this, and you want good
reliability;  Microchip does make pretty robust chips.  Add a nice heat
sink if over the max rated speed for that IC, probably.  Expect some
chips not to work at higher speeds, if rarely.  Worry less about a chip
run within it's rated speed range (i.e. an F84-04 run at 10MHz) than
outside the available speed range (i.e. an F84-04 run at 20MHz.)

And, think safety - We all know Murphy's Laws.  (My fathers' pellet
stove feeds pellets happily, even if the stove's gone out - we've had to
dig the entire firebox free of literally gallons unburned pellets, a few
times, not a lethal problem, but really QUITE annoying!)  If your
overclocked PIC fails, what'll the consequences be?  Annoying or
catastrophic?

Probably best to do the same job in a sneakier way with lower speeds, or
with different hardware (FPGA/CPLD/other front end) instead of pushing
hardware beyond it's limits, if you can.  The "Principle of least
astonishment" is here on Earth for a good reason, folks <G>

 Mark

Tobie Horswill wrote:
{Quote hidden}

--
I re-ship for small US & overseas businesses, world-wide.
(For private individuals at cost; ask.)

2000\03\22@171654 by briang

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In-Reply-To: <000c01bf9434$87b72440$spam_OUT0100a8c0TakeThisOuTspamvideotron.ca>

Tobie Horswill <.....thorswilKILLspamspam@spam@VIDEOTRON.CA> wrote:
>     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 ?

There is no risk of damage, just risk that they won't be reliable.

Brian Gregory.
briangspamKILLspamcix.co.uk

2000\03\22@172529 by John Orhan

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Hi Tobie,
I have seen a circuit where a 4Mhz Z80 was clocked at 10Mhz with the
addition of a 10M parallel resistor across the Xtal. I don't know how/why
this would work but am dying to find out.

                                                       John

               {Original Message removed}

2000\03\22@183649 by Brandon, Tom

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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 removed}

2000\03\22@190210 by briang

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In-Reply-To: <4042F7084FCDD311834B00104BCA47700B86EE@MAIL2>

John Orhan <.....JOrhanKILLspamspam.....EDM.COM.AU> wrote:
> I have seen a circuit where a 4Mhz Z80 was clocked at 10Mhz with the
> addition of a 10M parallel resistor across the Xtal. I don't know how/why
> this would work but am dying to find out.

This doesn't make sense.

The 10M resistor can't have anything to do with the overclocking.

For a start you don't connect a crystal directly to a Z80. It needs an
external clock oscillator.

Brian Gregory.
EraseMEbriangspam_OUTspamTakeThisOuTcix.co.uk

2000\03\22@191044 by Arthur Brown

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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 <Tomspamspam_OUTPSY.UNSW.EDU.AU>
To: <@spam@PICLISTKILLspamspamMITVMA.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 removed}

2000\03\22@193226 by Don McKenzie

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John Orhan wrote:
>
> Hi Tobie,
> I have seen a circuit where a 4Mhz Z80 was clocked at 10Mhz with the
> addition of a 10M parallel resistor across the Xtal. I don't know how/why
> this would work but am dying to find out.

May have been an old trick of mine you read John.
It was fine for the Z80's and gave them a kick start, to do with start
up current from what I remember.
Don't know if would apply to PICs.

Don McKenzie    KILLspamdonKILLspamspamdontronics.com      http://www.dontronics.com

World's Largest Range of Atmel/AVR and  PICmicro Hardware and  Software.
Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

2000\03\22@202655 by John Orhan

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Hi Don,
You're quite right now that I recall, it was your Pbuff/development kit.
Still got it too!!! Wonderful machine.

                                       John

               {Original Message removed}

2000\03\22@220324 by Don McKenzie

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Brian Gregory wrote:
>
> In-Reply-To: <4042F7084FCDD311834B00104BCA47700B86EE@MAIL2>
>
> John Orhan <RemoveMEJOrhanTakeThisOuTspamEDM.COM.AU> wrote:
> > I have seen a circuit where a 4Mhz Z80 was clocked at 10Mhz with the
> > addition of a 10M parallel resistor across the Xtal. I don't know how/why
> > this would work but am dying to find out.
>
> This doesn't make sense.
>
> The 10M resistor can't have anything to do with the overclocking.
>
> For a start you don't connect a crystal directly to a Z80. It needs an
> external clock oscillator.

Hmmm... Quite correct. Now I am showing my age. But I remember going
through this 10Meg across the xtl thing to kick start the xtl
oscillations. Now where was it?

Don McKenzie    spamBeGonedonspamBeGonespamdontronics.com      http://www.dontronics.com

World's Largest Range of Atmel/AVR and  PICmicro Hardware and  Software.
Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

2000\03\22@220944 by Don McKenzie

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Brian Gregory wrote:
>
> In-Reply-To: <4042F7084FCDD311834B00104BCA47700B86EE@MAIL2>
>
> John Orhan <TakeThisOuTJOrhanEraseMEspamspam_OUTEDM.COM.AU> wrote:
> > I have seen a circuit where a 4Mhz Z80 was clocked at 10Mhz with the
> > addition of a 10M parallel resistor across the Xtal. I don't know how/why
> > this would work but am dying to find out.
>
> This doesn't make sense.
>
> The 10M resistor can't have anything to do with the overclocking.
>
> For a start you don't connect a crystal directly to a Z80. It needs an
> external clock oscillator.

OK, I had to do an archival search of my old hard drive and found the
mention of the 10 Meg resistor.
Who needs a good memory, when you have a search command. :-)
================================================================
Most 84s can be speed up to 8Mhz, but note that this doesn't
comply with manufacturers specs. Tin canned, 4 legged TTL Oscillators
may
speed it along at 12Mhz too, but watch out for internal EEPROM
read/write
cycles! Gary Barnes of Western Australia put me onto the 12Mhz
Oscillator
trick.

Other users have told me that you can get some 4Mhz PICs running at 16
and
20Mhz by just putting a 10Meg resistor across the faster cyrstal.

Don McKenzie    RemoveMEdonspamTakeThisOuTdontronics.com      http://www.dontronics.com

World's Largest Range of Atmel/AVR and  PICmicro Hardware and  Software.
Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

2000\03\27@125709 by briang

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In-Reply-To: <38D98A2F.D4239E0AEraseMEspam.....dontronics.com>

Don McKenzie <EraseMEdonspamDONTRONICS.COM> wrote:
> Other users have told me that you can get some 4Mhz PICs running at 16
> and
> 20Mhz by just putting a 10Meg resistor across the faster cyrstal.

I would expect most PICs to work at these frequencies without any extra
resistor because the 4 and 10MHz parts are exactly the same as the 20MHz
parts and just  haven't been tested at 20MHz.

I can't believe a 10M resistor is going to make much difference. PICs
already have an internal resistor to bias them into the linear region.

If you really wanted you could hook up an oscilloscope and try different
resistors and see which value gave the strongest oscillations.

It'd be a whole lot simpler to just buy a 20MHz PIC though.

Brian Gregory.
RemoveMEbriangEraseMEspamEraseMEcix.co.uk

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