Hi,

A couple of weeks ago, I posted some questions that arose in my attempt to
use the PortB IOC feature interfacing with a quadrature rotary encoder.
Several of you responded with helpful comments -- thank you all for taking
the time to respond.

Among the responses, Brian Harris pointed me to AN696, which contains the
description of a perfect implementation for my application.  It uses a pair
of D-flipflops to generate 2 separate pulse streams, Up and Down.  These 2
streams are used as the clock inputs for TMR0 and TMR1.  So, neither
sampling nor interrupts are required.  I've studied the document, and simple
as it is, I don't understand part of it.

Here are some relevant excerpts from AN696:
"... Timer0 and Timer1 are used to accumulate the up and down pulses that
are derived from the encoder output signals.  The counters are never cleared
to avoid the possibility of losing count information.  Instead, the values
of the Timer0 and Timer1 registers saved during the previous sample period
are subtracted from the present timer values, using two's-complement
arithmetic.  This caclulation provides us with the total number of up and
down pulses accumulated during the servo update period.  The use of
two's-complement arithmetic also accounts for a timer overflow that may have
occurred since the last read."

And, here are the relevant C instructions to implement the scheme (to save
space:

int
 mvelocity, Dncount, Upcount;
long
 mposition;

void UpdPos(void)
{
 mvelocity  =  Dncount;  /* the previously-saved pulse counts */
 mvelocity -= Upcount;

 Upcount  = ReadTimer0();  /* the current counts */
 Dncount  = ReadTimer1();

 mvelocity += Upcount;
 mvelocity -= Dncount;

 mposition += (long)mvelocity << 8;
 }

Finally, my questions:  What in this code accounts for the description's use
of the phrase, "using 2's-complement arithmetic"?  And, how does this,
"account for a timer overflow that may have occurred ..."?

Stepping through the code, using decimal arithmetic, I don't get the
expected results.  Obviously, I'm missing some very important concept.

Can someone please explain how this works?  I'm baffled.

Al Young
Phoenix

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