Anybody got a good averaging algorithm?

I've got a stream of numbers from 0 to 15 coming out of a PIC '620
comparator representing temperature of an appliance.  When I turn on
the flourescent lamp on my bench, the program picks up a spike, and
the relay in this thing fires momentarily, because it reads the
noise.  "NO PROBLEM" I sez, and sat down to write an averaging
algoritm.

This ought to be so easy.  The first "Algorithm" anybody learns in
kindy-garden Math 1 is how to average numbers.  I've been checking
out verious ways of averaging a stream of data, though, and don't
have one that satisfies on a PIC with Two byte arithmetic and no
floating point math.

Try this.  Make a spreadsheet with 300 or 400 random integers between
0 and 16.  Take the arithmetic average of all of 'em, probably
around 8.   Then try to simulate a PIC program processing each number
in succession that would get near the average, without floating point
math.  I tried a half dozen ways this morning.

One trick I worked out is that the average of N numbers, given the
NTH piece of data is being processed now, is (N-1)*(old average)/N
plus (Nth piece of data)/N.  If you pick 256 data points, given the
old average AVG and the new data point DATA,

NEWAVG = {(AVG*255)+DATA}/256

That oughta work!  Not so fast.  Using only integer values,no
floating point math,  the average climbs slowly toward zero and stays
there.  PICs can probably do floating point math, but there's gotta
be an easier way!


Best Regards,

Lawrence Lile