Ripple Counter

A good first thought for making counters that can count higher is to chain Divide-by-2 counters together. We can feed the Q out of one flop into the CLK of the next stage. The result looks something like this:

The ripple counter is easy to understand. Each stage acts as a Divide-by-2 counter on the previous stage's signal. The Q out of each stage acts as both an output bit, and as the clock signal for the next stage.

We can chain as many ripple counters together as we like. A three bit ripple counter will count 23=8 numbers, and an n-bit ripple counter will cound 2n numbers.

The problem with ripple counters is that each new stage put on the counter adds a delay. This propagation delay is seen when we look at a less idealized timing diagram:

Now we can see that the propagation delay does not only slow down the counter, but it actually introduces errors into the system. These errors increase as we add additional stages to the ripple counter.


 

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