Counter Shift Registers
Shift register counters are a sequence of a certain number of core registers that are connected together to provide a clock-driven data shift. The most common of these are called Johnson counters and ring counters. Because the data changes in response to a clocking signal, they are used to determine the different time or state. For ring counters, they have a certain number of registers in sequence with the output of the last connected back to the first. A single high value is passed from register-to-register and then restarted again at the first. So for a simple 2-register ring counter, a 1 would pass between each circuit at every clock signal. For a 4-register counter, a 1 would pass through 4 registers before being sent back to the first - 1000, 0100, 0010, 0001. For these counters, the initial 1 state for the first register must be directed via external circuitry. A Johnson counter is nearly identical to a ring counter except it can hold twice as many states with the same amount of registers. It does this by inverting the signal in the last register as it returns to the first. It also has the advantage of not needing an external signal to activate itself, and it begins to count right away regardless of any signal provided internally. A 4 register Johnson counter sequence would be 0000, 1000, 1100, 1110, 1111, 0111, 0011, 0001. Read more Read less
The BOM Tool is available only for registered users.
Please Login or Create an Account to access the BOM tool.