Glenn said:
Why is it that some chips just don't seem to have any logic to their
pinouts. For example, why do they split up ports on microcontrollers? Or
on a BCD 7 segment display driver, why are a through e in order but f & g
are on the other side of a?
Ah, sweet mysteries of life.
I've always been annoyed by the fact that PIC microcontroller chips have
their power pins in the wrong places. For the PIC12F675, for example,
Vdd is pin 1, whereas Vss is pin 8. For an 8 pin PDIP package, ground is
usually pin 4, and Vdd pin 8.
Some chips use pin placement in an attempt to deal with heat. For
example, the TI SN754410 quad 1/2 H driver chip has 4 ground pins in the
center, which double as heatsinks.
I've also heard that some of the AC chips have unique placement of power
and ground in order to mitigate ground bounce problems, which can occur
when all the outputs go up and down simultaneously. Since they are so
fast, they can induce an oscillation on the ground plane that can affect
other chips in the vicinity.
I'm sure most of the placement of pins for logic families is historical.
I believe certain chips (some earlier cmos 4000 series) have fallen out
of favor because of pin placement, and been replaced with chips that
have more 'standard' placement.
I had problems with the f,g thing on a recent project. It would have
been so nice to have a single sided PC...
I'm suprised there aren't CPLDs targeted towards prototyping/hobbyist
use. Seems like using this technology, you could replace nearly any
logic chip with a faster version that has somewhat more flexible
pinouts. However, atmel and the other cpld mfgrs have proprietary
programming protocols that forces one to buy expensive programmers. That
makes it hard for people to do on the cheap in their garage.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.