> When we say that it is a PIC of 12,16,17 or 18 series, is it that we
> actually mean to differentiate it with the memory bit it can address at a
> time. Example PIC 18 series can fetch and process 16bit instruction at a
> time, similarly PIC 12 series executes 12 bit.
> In this case how many bits a PIC instruction would be(8,12 or 16)bits? Is
it
> possible that an application developed for PIC 12 could execute on PIC 16
> series. And what exactly we mean when we say PIC can address 16 bit at a
> time? Is the memory addressed in RAM or ROM are they talking about?
> Regards,
> Satya Prakash Prasad

All PICs address data memory 8 bits at a time. Instructions vary in width
based on the series. Wider width allows more RAM to be addressed in each
'bank' and more versatile instructions.

PIC12 program port very easily to PIC16. Mostly the PIC16 is a direct
superset of the PIC12. Instructions in the PIC12 are 12 bits each, in the
PIC16 they are 14 bits. Note that some small PICs that are labeled PIC12 are
really PIC16 inside. RAM banks are 16 bytes each.

PIC17 is basically a forgotten attempt at an improvement on the PIC16.
Although there are many applications using it (included MChips' own PicStart
Plus), few people would choose it for a new design. It is again possible to
port from the PIC12 or PIC16 to the PIC17. The PIC17 uses 16 bit
instructions. RAM banks are 128 bytes each.

PIC18 is a very large improvement over the three prior series. It was
designed to more readily support C compilers. Porting from PIC12 or PIC16 to
PIC18 is pretty straightforward. It is a little trickier moving from the
PIC17. In all cases, there are some gotchas regarding what happens to flags
on certain instructions. Like the PIC17, the PIC18 uses 16 bit instructions.
RAM banks are 256 bytes each.

Note that all cases of 'porting' require modifying the source code and
rebuilding the executable.

Bob Ammerman
RAm Systems

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