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Registered Member #3546
Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18
I have this terrible tendency to try to make things not suitable for 8-bit PICs. I also only program PICs in assembly; I know what you're going to say.....
Anyways I have a small problem. When I write my programs I have a tendency to go over 256 lines of code. This is fine because from my understanding the PIC16F8xx family that I typically use has four times this code memory. Unfortunately if your code goes into one of the later program code banks and you attempt to adjust the code line for something such as a look up table it throws me into that location in the first bank. How do I ensure that I stay in the second or third or four memory bank without having to leave all my look up tables at the top of my code. Honestly I asked my professor that question and he didn't know how, hopefully one of you guys has figured this problem out in the past.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Lookup tables that cross page boundaries are a classic PIC f***-up. Using the ORG statement helps, but you also have to set the high bits of the program counter appropriately.
Once you've started playing with PCLATH, you also have to save it and restore it in your ISR, if you're using interrupts. (Took me a long time to figure that one out.)
Here's part of a lookup table routine I wrote earlier, for a keyboard scanner implemented on a PIC16F870. It shows most of the techniques you need for a reliable LUT, even a simple bounds check with the "andlw 0x3f". (the table is 64 entries long)
To clarify, you can place this code in any bank and call it from any other bank using "call kblut". The call instruction pushes the full program counter on the stack, not just the low 8 bits, so the page you were in gets remembered. The "retlw" instruction in the table will send you back to the one after the "call".
This one doesn't take account of page boundaries within the table itself. So it's limited to about 250 entries, assuming you position it at the beginning of a page with the org directive. Microchip have an app note somewhere that shows how to do longer ones.
Registered Member #3546
Joined: Tue Dec 28 2010, 08:24AM
Location: Pittsburgh, PA
Posts: 18
I didn't realize that you could simply add org statements into the code like that, or at least I have never seen that done before? I figured it had something to do with the PCLATH register too but I just wasn't sure how to use it. Thank you for that example McConner.
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8-Bit PIC Code Memory
