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Chapter 5. Sequential Machines > Two-Phase Systems for Latches

5.3.3. Two-Phase Systems for Latches

multiple ranks of latches

A single rank of flip-flops cutting the system’s combinational logic is sufficient to ensure that the proper values will be latched—a flip-flop can simultaneously send one value to its output and read a different value at its input. Sequential systems built from latches, however, are normally built from two ranks of latches. To understand why, consider the relationships of the delays through the system to the clock signal that controls the latches, as illustrated in Figure 5-16. The delay from the present state input to the next state output is very short. As long as the latch’s clock is high, the latch will be transparent. If the clock signal is held high long enough, the signal can make more than one loop around the system: the next state value can go through the latch, change the value on the present state input, and then cause the next state output to change.


  

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