11.1. Binary count sequenceIf we examine a fourbit binary count sequence from 0000 to 1111, a definite pattern will be evident in the "oscillations" of the bits between 0 and 1:
Note how the least significant bit (LSB) toggles between 0 and 1 for every step in the count sequence, while each succeeding bit toggles at onehalf the frequency of the one before it. The most significant bit (MSB) only toggles once during the entire sixteenstep count sequence: at the transition between 7 (0111) and 8 (1000). If we wanted to design a digital circuit to "count" in fourbit binary, all we would have to do is design a series of frequency divider circuits, each circuit dividing the frequency of a squarewave pulse by a factor of 2:
JK flipflops are ideally suited for this task, because they have the ability to "toggle" their output state at the command of a clock pulse when both J and K inputs are made "high" (1):
If we consider the two signals (A and B) in this circuit to represent two bits of a binary number, signal A being the LSB and signal B being the MSB, we see that the count sequence is backward: from 11 to 10 to 01 to 00 and back again to 11. Although it might not be counting in the direction we might have assumed, at least it counts! The following sections explore different types of counter circuits, all made with JK flipflops, and all based on the exploitation of that flipflop's toggle mode of operation.
