Jacek,

Please excuse the mistake I made in the previous post. For proper scaling, the difference equation should read -

S(n)=p(n)+p(n-1)+p(n-2)+p(n-3)

After sending S(n) to the display, move p(n-2) to p(n-3), p(n-1) to p(n-2) and
p(n) to p(n-1). Then, when the next 15 second period is complete send the counter increment to p(n), perform the sum, display the sum and repeat the cycle.

Bruce