Those wacky Microchip Designers!

[Ron Marcus]

I guess this is a bit of a gripe, but maybe there is a legit answer to this question. I seem to be working with more and more pins on a single processor these days. Why do they split some of the ports? Fer instance, on the 18F4520 in the TQFP, ports C and D are split right down the middle. Is there a legit reason for this, or more convenient in the die fabrication?
Ron

[Darrel Taylor]

I can't say how it ended up that way in the first place, but I think at this point it's just a matter of compatability with previous designs.

The 18F4520 is based on the 18F452 who's pinout was made to be compatible with previous 44pin devices such as the 16F877.

Within each group, there are many similarities.
ie. Many 18-pin devices share the same pin configurations as well.

_

[T.Jackson]

I agree with that - possibly for compatibility reasons to help make the switch across as painless as possible.

[skimask]

I guess this is a bit of a gripe, but maybe there is a legit answer to this question. I seem to be working with more and more pins on a single processor these days. Why do they split some of the ports? Fer instance, on the 18F4520 in the TQFP, ports C and D are split right down the middle. Is there a legit reason for this, or more convenient in the die fabrication?
Ron

Ever noticed how you can ALMOST put a 28 pin DIP PIC into a 40 pin DIP PIC socket? Or for that matter, you could (again ALMOST) put an 18 pin PIC into a 40 pin PIC socket.
Same thing for the TQFP. With the 18F672x (64pin TQFP) and the 18F872x (80pin TQFP), everything is in the same place except for the 16 corners pins. I used that to make my 64/80 TQFP adapter board, which has a 40 pin IDC ribbon cable header to plug a 672x/872x into a socket previously occupied by 877, 452, and a 4620, but with loads more memory. And I use another header holding the rest of the pins to run another PCB.

[mister_e]

In general it should be true... we know Microchip ;) You can also fit most 28dip in the same 28dip socket as well... i said most... 18F2431 don't have RA5(or else I/O) ... but Vcc ... dear Microchip :D