PCLATH

PIC Micro: Handling Memory Boundaries

I have written a lot of code for Basic Stamp and PIC and continue to learn with each and every project. Recently I had what may seem like a simple problem to most people writing PIC code but in researching for an answer I found quite a bit of difference in opinion as well as incorrect information on many websites.

Before going further I recommend that anyone serious about developing 
embedded controls using the Microchip PIC micros should join the MIT PICLIST. 
The folks on this site are wonderful. Many are full time developers in this field.

Calling lookup tables across memory boundaries.

I recently had a project that basically was a programmable timer / actuator.
I designed this timer to cycle a solid state relay. All parameters such as 
the number of cycles, how long it was on, how long it was off an so on. I used
nearly every available bit of space in a 16F628. I knew of all the warnings 
concerning placing my lookup tables at the beginning of program memory but
I had numerous lookup tables because I used programming prompts on an LCD. 
Well, with the help of a local PIC enthusiast and very little from the 
internet (PICLIST had much information but somewhat confusing at least for me).
he gave me a very direct and simple way to keep my lookup table calls from 
getting lost so to speak. The following is a brief example of what I did in my
project and I hope it will help you in the future.

When calling a lookup table look to see in what memory page it is located in.
The PIC needs to know that once it makes a jump using the PCL you need to 
remind it so to speak where it is located in reference to the memory pages.

If for instance you call a table from memory page 2 to a lookup table in page 1
at the beginning of the table do the following:

PRG_MSG_ALSEC
				BCF			PCLATH,0
				BCF			PCLATH,1
				BCF			PCLATH,2
				ADDWF		PCL,F
				DT			"ALARM DUR SECS",0

PCLATH 0 - 2 in this instance is set for page 0 if this were located in page 1
it would look like this.

PRG_MSG_ALSEC
				BSF			PCLATH,0
				BCF			PCLATH,1
				BCF			PCLATH,2
				ADDWF		PCL,F
				DT			"ALARM DUR SECS",0

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