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Step 4 - Insert in the Required Gates
Right Before the T
Now that we have the information as to what gates we need to use from the comparison table, we can modify the corresponding stages as indicated by each column. For this to work correctly, we would want to insert our gates so that the output of our newly found gate combination is wired directly to the input of the stage it is meant to modify.
Because it has been some time since we last saw the two available gate combinations needed to implement the comparison table, it would be advantageous to review them.
EXAMPLE:
Show where the gates, from its comparison table, are to go in a mod-25 counter.
For a mod-25 counter, we would need 5 bits since that is the minimum number of bits needed to count up to the final count of 24. Laying out five T-flip flops and connecting them together as if we were creating a generic mod-2^5 counter yields the following: (The way the wires are routed should give a hint as to where I am going to insert the gates)
Plugging in the values needed for the comparison table and using the steps specified by the previous page gives us the following table.
For clarification, a modified copy of the counter we have so far is marked so that it is clear as to where the gates should be placed. Note the location of the LSB and MSB stages of the counter relative to the location of the LSB and MSB on the comparison table.
Starting from the LSB side, we shove an INV and AND gate between the +5v high line and the T-input of the first stage. The trailing input of the INV gate will be addressed on the next page since we are only interested in, for now, the gates given by the comparison table.
Proceeding onward from the LSB, the next two stages do not need to have any gates added to their input because the comparison table shows dashes for them. However, the third bit of the comparison table requires that an OR gate be inserted in front of its respective T-flip flop on the counter schematic as follows.
Similarly, the MSB requires an OR gate in front of it as well to complete this exercise.
EXAMPLE:
Show where the gates, from its comparison table, are to go in a mod-10 counter.
For a mod-10 counter, we would need 4 bits since that is the minimum number of bits needed to count up to the final count of 9. Laying out four T-flip flops and connecting them together as if we were creating a generic mod-2^4 counter yields the following:
Plugging in the values needed for the comparison table and using the steps specified by the previous page gives us the following table.
Starting once again from the LSB, we find that the initial flip flop requires no special attention since the comparison table has a dash for its place. However, upon moving onto the second flip flop, we find that we would need to put an INV-AND gate combination is required between its T-input and whatever is currently feeding into it.
The next stage requires, like the first, that no gates are needed. Finally, the last stage or MSB requires an OR gate to be inserted in front of it to finish this example.
