Re: [avr-gcc-list] Best practice for external variable (newbie-ish question)

[David Kelly]

On Nov 20, 2005, at 7:29 PM, Matthew Smith wrote:

> Thanks for that; I do have everything in one file and I'm getting some
> very, very odd and unpredictable actions.  The system is a pump
> controller that reads pressure via an ADC and uses the AVR timer to
> determine a minimum run time (unless a critical pressure is  
> reached, in
> which case the timer is ignored).
>
> I've got an LED showing me when the timer is running and another on a
> solid state relay showing what the pump is supposed to be doing.
> Substituting a potentiometer for the pressure transducer, I've  
> observed
> it working as planned, but sometimes the timer never expires (or won't
> start) and sometimes the "virtual" pump turns on, but never off.

Behaves with potentiometer but not with real sensor? Then I suggest  
studying the analog input circuit from the sensor and consider adding  
filtering.

One might debug the A/D by spitting the read values out a serial port  
but this consumes enough CPU cycles that it might affect the results.  
If using a debugger then one might modify the code to stash the last  
128 samples in a circular buffer then when one observes misbehavior  
use the debugger to break and study the buffer contents.

#define BUFFER_SIZE (1<<7)        // must be a power of 2, (1<<7) is 128
uint8_t buffer_index = 0;       // global is easier for debugger to find
uint16_t buffer[BUFFER_SIZE];

void buffer_it( uint16_t value )
{
        buffer[buffer_index++] = value;
        buffer_index &= BUFFER_SIZE-1;
}

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.