> -----Original Message-----
> From: Patrick B. Murphy [SMTP:luke631@MTS.NET]
> Sent: Wednesday, September 03, 2003 2:47 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC:] How to monitor a dozen shafts?
>
> I've never tried to measure periods by monitoring the inputs. In the
> few projects where I have counted pulses, for example, where I
> monitored a flow meter, I used the Capture or Compare module. I want
> to be sure I understand what you mean here, so I want to think about
> this for a while longer. I have a hunch that this type of monitoring
> would be easier in assembly than in CCS C because of the timing
> loops.
>
It realy boils down to the precision you need.  The period of just one tooth
on the 16 tooth wheel at 200RPM would be a whopping 188ms, so you wouldn't
need to poll the inputs particularly quickly to get a reasonable precision.

I don't think you will need to resort to assembly to get a useable result,
especialy with a 20MHz clock.  Simply set up an interrupt (the CCP module is
great for arbitrary rate interrupts with no timer loading overhead) and then
check the state of each input. By comparing the current reading to the
previous one, you can detect a rising edge, falling edge or static high or
low.  By running a counter for each input and storing the value on a rising
(or falling) edge you can easily and quickly make a multi channel counter.

Something like the following should do the trick (only 8 channels, but gives
you the idea).  You would probably want to add some simple filtering and
some code to detect a rollover of the counters etc.  As it stands the code
executes in around 170 cycles excluding context save etc. (compiled with
HiTech PICC).  At 20MHz that's just 34us, I would think the 12 channel
version with the bells and whistles could be fairly easily made to take
under 100us, so a 256us interrupt would be possible with a decent amount of
cpu time left for your main loop.

HTH

Regards

Mike


unsigned int period[8]; // array to hold period for each input.

void interrupt isr(void)
{
static unsigned int counter[8];
static unsigned char prevState;

unsigned char inState;
unsigned char pulse;
unsigned char chan;
unsigned char mask;

        // check for CCP interrupt
        if(CCPIF && CCPIE)
                {
                // reset interrupt flag
                CCPIF = 0;

                // grab snapshot of inputs
                inState = PORTB;

                // find any rising edges
                pulse = (inState ^ prevState) & inState;

                // check each channel for a rising edge (bit set in 'pulse')
                mask = 0x80;
                for(chan = 7; chan--; )
                        {
                        if( pulse & mask )
                                {
                                // rising edge found so load period array
with the count value
                                period[chan] = counter[chan];
                                // reset the counter
                                counter[chan] = 0x00;
                                }
                        else
                                {
                                // no rising edge found so increment counter
for this channel
                                counter[chan]++;        // should check for
rollover here
                                }
                        // shift the bit mask to the next input bit.
                        mask >>= 1;
                        }
                // save current port state for next comparison
                prevState = inState;
                }

}





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