Structure
There are basically two types of logic gate - a '1' gate and a '3' gate. These differ only in the clock phases used to drive them. A gate can have any logic function; thus potentially each and every gate has a customized layout. An example 2-input NAND 1 gate and an inverter 3 gate, together with their clock phases (the example uses NMOS transistors), are shown below:
The ø1 and ø3 clocks need to be non-overlapping, as do the ø2 and ø4 clocks. Considering the 1 gate, during the ø1 clock high time (also known as the precharge time) the output C precharges up to V(ø1)-Vth, where Vth represents the threshold of the precharge transistor. During the next quarter clock cycle (the sample time), when ø1 is low and ø2 is high, C either stays high (if A or B are low) or C gets discharged low (if A and B are high).
The A and B inputs must be stable throughout this sample time. The output C becomes valid during this time - and therefore a 1 gate output can't drive another 1 gate's inputs. Hence 1 gates have to feed 3 gates and they in turn have to feed 1 gates.
One more thing is useful - 2 and 4 gates. A 2 gate precharges on ø1 and samples on ø3:
and a 4 gate precharges on ø3 and samples on ø1.
Gate interconnection rules are: 1 gates can drive 2 gates and/or 3 gates; 2 gates can drive only 3 gates, 3 gates can drive 4 gates and/or 1 gates, 4 gates can drive only 1 gates:
Read more about this topic: Four-phase Logic
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