16F877A to 18F452 Migration

Charlie U · Posted: Sat Jan 31, 2004 10:27 am

I am working on a small hand-held device which has very little pcb space available for the electronics. The feasability phase of the project used the 16F877 and I am hesitant to change. However, there is this looming problem on the horizon of "not enough code space". I am being asked to start the pcb layout before the code is completed and thus the processor selected. Here's the problem: to save space, we may need to use the QFN package, since it is less that half the area of the TQFP. In most, if not all, other packages, Microchip has kept a consistent pinout between the 16F877A and the 18F452, with the exception of the programming pin. But in digging through the errata sheets (which is the only place I can find complete QFN details) it appears that they have changed the pin out between the 16F877A and 18F452 in the QFN package.

Has anyboby worked with the QFN packages and are the pinouts truly different between the 16F877A and the 18F452? Also, I am not using the comparator in the 877A so are there any other programming subtleties that I should be aware of, beyond what's been posted here in the past?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Charlie U · Posted: Sun Feb 01, 2004 4:38 pm

Thanks for taking the time to review this, I truly appreciate it!!! You are absolutely right.

I can't believe that I had my head that far you know where!!!

I have been looking at these pinouts for far too long and I think I must have confused the QFN package on the 18F452 with the QFP package on the 16F877A.

This is a HUGE relief. I can now forge ahead knowing that the parts are almost interchangable with the exception of the PGM pin (RB3 for the 877A and RB5 for the 452).

Thanks again.

Al · Joined: 13 Nov 2003 Posts: 28 Location: Belfast

I appreciate that you may ultimately have to make the move to the bigger chip, but there is quite a bit you can do to make more efficient use of memory. I don't know if you are close to your ROM or RAM limits but you can improve both. Look up the Compiler help under Contents
then look at Common Questions and Answers and look at the question What can be done about an OUT OF RAM error?. Also look at Error Messages->Compiler Error Messages->Not Enough Ram for all Variables.. It is a useful excercise.
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Alan Murray