David VanHorn wrote:

>>When I use a PIC, I want real-time performance.
>>    
>>
>
>
>Must resist AVR jab.. MUST resist!..
>
>So I use interrupts
>  
>
>>for anything external and a loop in the main program that checks to
>>see if any of my interrupt routines need more processing. I only
>>use that if my interrupt routine is going to take too long and then
>>it just does the minimal amount of processing necessary and sets
>>a flag for the main loop.
>>    
>>
>
>  
>
Agreed. The interrupt has always been used by me to set flags, which are 
then served in
a prioritized manner by a main loop. The ONLY exception is in the case 
of a serial characters
just received. I even service the serial outbound port in a main loop.

Almost everybody uses a "tick" timer, such as tmr0, to set flags at a 
precise interval. If possible, make
the tick interval as large as possible to reduce interrupt overhead.

Don't waste a good interrupt on non-essential tasks. For example, the 
MSSP port rarely
needs to use its interrupt, nor should ADC tasks, nor should many others.
.
To test for the possibility of overrun (missing a "tick" due to heavy 
interrupt processing),
set a port pin when you enter the interrupt and clear it when leaving, 
then examine it
with an oscilloscope while running. If your pin mormally stays true more 
than 25% of the
 time, you may have a problem when very infrequent items occur.

The most common interrupt error made is making too many changes in the 
same interrupt.
For example, most people have a task that occurs when counterA counts 
down to 0, and
another when counterB counts down to zero, etc.  But if they are chained 
together, there
will be a situation where all the tasks will happen in the same 
interrupt.  The best way to
solve this is to stagger the tasks by jump tables so that they CAN'T 
occur at the same time.

This is as multitasking as one can get with PIC16. But it works well.

--Bob

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