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State Machine Implementation using CCS

 
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AnthonyF
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State Machine Implementation using CCS
PostPosted: Wed Oct 10, 2007 3:34 pm     Reply with quote
I am using CCS with a PIC 16F877. I am implementing a state machine, in this application the state variable is a byte, so 255 possible states.

Right now omitting #defines it would look something like this:

Code:
switch(state){

case 0x01 :
   command_01();
   break;
   
case 0x02 :
   command_02();
   break;

...

case 0xFF :
   command_FF();
   break;
}


I am trying to optimize this logic. I am familiar with x86 assembly and in assembly I would do something like:

mov register, state
jmp register
jmp command_01
jmp command_02
jmp command_03
...
jmp command_FF

This avoids up to 255 comparisons.

....Any suggestions on how to do this using the CCS compiler and a PIC16F877?
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Wed Oct 10, 2007 3:41 pm     Reply with quote
Here are some threads about switch-case and if-else efficiency.
http://www.ccsinfo.com/forum/viewtopic.php?t=28643
http://www.ccsinfo.com/forum/viewtopic.php?t=21373
http://www.ccsinfo.com/forum/viewtopic.php?t=20721
http://www.ccsinfo.com/forum/viewtopic.php?t=17868
http://www.ccsinfo.com/forum/viewtopic.php?t=28970
guestplanet
Guest
PostPosted: Wed Oct 10, 2007 9:15 pm     Reply with quote
removing the default statement from switch-case, compiler will then generate a 'table jump' codes but this in turn will cause your microcontroller to run an approximate 30 instructions before executing functions within each case
AnthonyF
Guest
PostPosted: Fri Oct 12, 2007 3:17 pm     Reply with quote
Thanks,

Looks like if you are comparing with constants as I am the lookup table is hard coded exactly as I want requiring minimal processing during each run.

004C: GOTO 01D
004D: GOTO 024
004E: GOTO 02B
004F: GOTO 032
0050: GOTO 039
0051: GOTO 047
0052: GOTO 040
guestplanet
Guest
PostPosted: Mon Oct 15, 2007 6:48 pm     Reply with quote
I think we all would like to have minimal processing before executing each task.
how do you propose to code 'a table read using computed GOTO method' in C?
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