Digital Design—Sequential Logic

Authored by: Sin Ming Loo , Arlen Planting

The Industrial Electronics Handbook

Print publication date:  March  2011
Online publication date:  April  2016

Print ISBN: 9781439802793
eBook ISBN: 9781439802809
Adobe ISBN:


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Memory is the difference between combinational and sequential logic. With combinational logic, the output is generated from inputs (after some gate propagation delays through the logic gates). Propagation delay is the length of time from when the input to a logic gate is valid, to the time when the output of that logic gate is valid. The propagation time will determine how soon a new set of inputs can be processed with the output correctly generated. A sequential logic design has memory elements. The output is dependent on not only the current inputs, but also on the state. This means that sequential logic needs to know where it was and where it is now in order to determine where it is going with the current set of inputs. For example, a full adder is a combinational logic component as it adds input A and B to produce the sum. However, an up-down counter is a sequential logic component because the counter knows the current value; depending on the up-down control input, it will determine the next count value. Figure 21.1 shows an illustration of the differences between combinational and sequential logic.

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