Maximum frequency output from a PIC

[rsocor01]

Henrik,

I don't have the definitive answer either, but I think that what Art is suggesting makes sense to me. I have tried moving to the beginning of the program subroutines that I use a lot in the program and the used programming code space is reduced by a good number of bytes. So, I assume that the program is also executing faster. I believe that this has something to do with the way the program memory is organized in pages in many PICs.

Robert

[HenrikOlsson]

Hi,
On the 12F683 and 16F819 there is only one page of program memory so it doesn't (or shouldn't if my reasoning is correct) matter where you put anything because there will never be a need to switch pages.

If I understand what Art wrote correctly he's basically claiming that the "further" the GOTO jump is, the longer it takes and the further "down the list" your subroutines are placed the longer it takes to GOSUB them because the program counter starts "searching for them" from the beginning of the program. I don't think that is correct - neither on PICs with a single memory page nor on those with multiple pages.

If using a PIC with the program memory spanning several pages and it has to jump (GOTO or GOSUB) from ona page to another then the compiler will insert code to make sure the correct page is selected before executing the GOTO but it either does that or it doesn't and that piece of code (to select the correct page) will be always take the same amount of time (a couple of cycles I imagine), it doesn't matter where or "how far" it is going to jump.

/Henrik.

EDIT: Robert, which PIC did you use in the project where you moved the subroutines?

[Art]

That is right sorry, I have my interpreted and compiled BASICs mixed up.

It's probably not bad practice anyway, so that your fast code is not kicked
out of the first page when you add picbasic's macro type commands such
as serin/out, DTMF, SOUND, pauseus, etc.
It also matters with C later on, to to avoid forward declarations.

It makes me wonder now, if every goto command also has the PCLATH
command with it, or if picbasic waits till the program extends past the 2K page (probably does).

You also turned off the watchdog, but didn't tell PBP not to insert clrwdt commands.
You would only need a single asm "@ clrwdt" inside the whole Main loop,
and calculate that into your subtract from the delay time,
but they are likely in the PAUSE commands, and factored into timing I think.

[HenrikOlsson]

Nope, not bad practice at all, I just don't see it making any difference in this application. But again, do try it - I might be wrong!

As for tweaking the delay time, it's a good thing to keep in mind for when timing is REALLY criticial but for this application, I don't know.... IMHO it's making things WAY more complicated than it is....

We're talking 20ms total period time here (ideal), 15ms 'carrier on', 5ms 'carrier off'. The PIC is running at 8MHz so each instruction is 500ns, it takes two instruction cycles to GOTO Main, add another couple of instructions for the rest, lets call it 20 in total - the actual period would be 20.01ms, an error in timing of 0.05%, I think that miniscule error will be swamped by the inacuracy of the internal oscillator. Of course, it MIGHT matter but then you need something better than the internal oscillator to run it.

/Henrik.

[rsocor01]

Henrik,

I think used a PIC16F727 when I did that. I think I got that idea of moving the most used subroutines to the beginning of the program when reading a post by Melanie. It had something to do with saving programming space in a PIC. I will try to find that thread later.

[rsocor01]

Ok, here is the post

http://www.picbasic.co.uk/forum/show...0527#post80527

[Art]

Henrik was right there, I was selling PBP short.
It shouldn't matter with PBP programs under 2K anyway,
but you'll appreciate the practice if you use another language.

You remember the old interpreted BASIC with line numbers?

10 CLRSCREEN
20 PRINT "HELLO WORLD"
30 GOTO 20

It's the goto in that one that searches a list to find the line/label it's after.
In this case (30 GOTO 20), it searches from the beginning, but if the destination
line is after the GOTO command, it searches from the GOTO command onward.
Some interpreted Pic BASIC language (not PBP) probably does the same thing
even if you don't need to use the line numbers anymore.

If you just want to check a byte variable is in BANK0 to access it without switching,
you can increment it with inline assembler.

@ incf _B1;

should be the same as

B1 = B1 + 1;

If byte B1 was not in BANK0, a different variable or some unassigned RAM will be incremented.