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Thread: 16F84A - 4MHz or 20MHz?
www.piclist.com/techref/microchip/devices.htm?key=16F
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face BY : David VanHorn email (remove spam text)



At 10:40 AM 11/4/00 +1300, Jinx wrote:
>>>The Product Line Card (for Qtr 2 2000) I received from Microchip shows
>>>the maximum frequency for 16F84A as 20MHz. But the marking on the
>>>chip is "PIC16F84A-04/P". Does "04" in the marking mean that its max
>>>imum frequency is only 4MHz?
>
>IIRC this was how it was explained to me some time ago. Unless something
>has changed, all PICs of the model are made from the same die, and each
>individual part is tested for speed. The -04 parts are guaranteed to run at
>4MHz, but may actually run much faster, perhaps even up to 20MHz. Or
>if there's a surplus of 20MHz parts on the market, -20 parts may be marked
>as -04 parts for purely economic/supply/inventory reasons. So unless you
>know why a part is marked as -04, you can't say for sure what its maximum
>operating speed will be. You could get lucky and have a -20 marked as a
>-04. Somebody claimed last year on the list to have overclocked a -10 to
>> 50MHz with no apparent problems


This sort of reasoning will get you in lots of trouble.

What you know from a -04 marking is that the device is guaranteed to
perform to spec up to that frequency. Beyond that, you're on your own.

The -04s may in fact be made on the same wafers as the -20s.
However, different parts of the wafer may perform better or worse than the
others.
This is why they test.

It's true that a given chip may run a given program at a given temperature
and VCC, at some higher clock speed.  However, you have no guarantee that
adding or removing a single instruction won't cause mysterious flakey
problems.

Debugging time is too precious to waste it trying to save a dollar on a
faster rated chip.

The approach I take for prototyping and hobby use, is to buy the fastest
grade chips.
Then, if a given project dosen't need the speed, I'll buy the lower grades
for production.  I'd rather save the $ on the bulk of the chips I buy,
rather than on the few I need for prototypes.


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