piclist 2002\09\12\075340a >
Thread: ; 4Mhz vs. 20Mhz PIC's.
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face BY : Russell McMahon email (remove spam text)



>  Is it possible to say, in a general way, if the 4 or the 20 Mhz
> version are the most popular? From one distributor, when asking
> how come they had only the 4 Mhz F628 on there WEB page, I got the
> answer that there wasn't much call for the 20 Mhz F628 (but
> they *can* sell it if you ask, of course).

I just priced 16F628's today from a local New Zealand supplier. The 20 MHz
parts were slightly CHEAPER than the 4 MHz parts. It seems a great shame not
to buy a part that will go literally 5 times as fast if the price is only
slightly more. No reason at all not to buy it when it costs slightly less
:-)

The general opinion is that 4 MHz parts and 20 MHz parts are drawn from
IDENTICAL production lots. They are tested for parameter match at full rated
speed and those which don't meet spec at 20 MHz but are OK at 4 MHz are sold
as 4 MHz parts. Also, if they have done a good job, most will meet 20 MHz
spec and they will mark some as 4 MHz parts even though they are 20 MHz
capable. Don't, of course depend on this ! :-) - YMMV.

> Then, there might be other issues when comparing 4 with
> 20 Mhz parts, such as power consumption. Is the power
> consumption of the 20 Mhz part running at 4 Mhz the same
> as the 4Mhz part running at "full speed" ?

It is quite possible that a 20 MHz part operated at 4 MHz may have superior
specs to a 4 MHz part operated at 4 MHz. Depends on what it was that caused
the 4 MHz part to not meet its full speed spec originally. One spec is max
current drain and this increases with speed so a 4 MHz part MAY be so marked
because it was a power hog at higher speeds.

I would ALWAYS buy 20 MHz parts if the price difference was anything like
reasonable as it gives you much greater flexibility. The price here in NZ in
hundreds is about $NZ4.50 (just over $US2) which is about the same as
Digikey charge in hundreds. Here they will sell tube quantities at the same
price if you talk nicely - I suspect that Digikey is immune to such
blandishments.

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