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Step 5 - Wire up to the Final Count
Find the Final Count
The "control signal" that we left hanging back on the last step is addressed by creating the logic for finding if the current state of the counters match that of the "final count". To do so, we look back at our comparison table and see that ANDing all the ones together that are present on the "final count" row for each corresponding bit will accomplish what we need. Simply stated another way, for any stage that has a one on the comparison table's "final count" row, we take its output to be the input to the new AND gate.
EXAMPLE:
Complete the mod-25 counter by wiring up the "final count" to the added gates.
First we look at our comparison table and we see that the AND gate for the final count should only need two inputs - one from stage 4 and the other from stage 5.
Next, we then take the outputs from the stages we have identified, AND them together, and feed the result to the inputs of the gates that we left hanging in the previous step.
Although we are technically done, there is one easy optimization that can be performed on the logic feeding the LSB stage of the counter. Since an AND gate's output is only true if both of its inputs are a logical one, the AND gate feeding the first stage can be simply thrown out and the inverter tied directly to the T-input.
This modification does not change the operation of the circuit because the inverter's output is a logical one when the "final count" has not been reached and thus the first stage is free to toggle.
And... we are finally done creating a mod-25 counter!
EXAMPLE:
Complete the mod-10 counter by wiring up the "final count" to the added gates.
First we look at our comparison table and we see that the AND gate for the final count should only need two inputs - one from LSB stage and the other from MSB stage.
Next, we then take the outputs from the stages we have identified, AND them together, and feed the result to the inputs of the gates that we left hanging in the previous step.
And... we are finally done creating a mod-10 counter!
