D/A 32 therefor produces at terminal 32a, as a function of time, the stair-step triangular-shaped waveform illustrated in FIG. 2A at a period 8???fC. The numbers 0, 3, 4, 7, 8, 11, 12 and 15 at the left of FIG. 2A (and FIG. 2B) are the decimal equivalents of the various binary numbers generated at the exclusive-OR gates 30a through 30d, although the numbers are generated in a sequence other than t