Why the strange pinouts?

G

Glenn Ashmore

Guest
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?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
 
"Glenn Ashmore" <gashmore@cox.net> wrote in message
news:gIV0e.72834$SF.14049@lakeread08...
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?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
Click to expand...
The typical reason is for the convenience of the die layout. Another reason
would be for economics, nobody would use a more expensive package that
is also larger and therefore used more PCB real estate. There are also
power dissipation reasons as well as electrical noise minimization. Much
depends on the function of the IC.
 
On Fri, 25 Mar 2005 10:00:35 -0500, "Glenn Ashmore" <gashmore@cox.net>
wrote:

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?
Click to expand...
---
Because you've got to bring all the inputs and outputs to the edges of
the silicon to connect to the lead frame, and most most times
(always?) you can't get all the I/O's to come out perfectly ordered
just the way you want them to. It's like laying out a single-sided
PCB. (At least it was back when I was doing that kind of stuff. I
don't know how many "layers" are available now though...)
---

on a BCD 7 segment display driver, why are a through e in order but f & g
are on the other side of a?
Click to expand...
---
Same story, I suspect.

In truth, it doesn't matter much since you're going to have to do a
PCB layout anyway, and even if all the I/O's came out of the chip
ordered perfectly, the layout would vary from PCB to PCB.

--
John Fields
 
Glenn Ashmore wrote:
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?
Click to expand...
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.
 
On Fri, 25 Mar 2005 11:08:05 -0800, Robert Monsen wrote:

I had problems with the f,g thing on a recent project. It would have
been so nice to have a single sided PC...
Click to expand...
You could have:
http://www.irctt.com/pdf_files/ZEROHM.pdf
;-)

Cheers!
Rich
 
On Fri, 25 Mar 2005 10:00:35 -0500, Glenn Ashmore wrote:

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?
Click to expand...
It's because of the way the ins and outs landed on the chip when they
designed it - you don't design a chip the way you design a circuit, and
stringing a conductor all the way around the die to come out at a certain
pin is a LOT different from doing it with a PCB trace. Rearranging inputs
and outputs would have made the chips more expensive.

Lately, they've been doing it since designs are so complicated that one
jumper or so here or there doesn't add that much to the cost.

Hope This Helps!
Rich
 
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