Page 97 - Handout Digital Electronics

LECTURE 12 SEQUENTIAL LOGIC CIRCUITS

12.1 Introduction
A sequential logic circuit is a digital circuit whose output depends on the following:

• Present inputs
• Present State
• Previous inputs

Sequential circuits have memory (storage) capabilities so they can somehow “remember” previous
inputs. See classical block diagram of a sequential logic circuit.

The D flip flop make up the storage section. From the block diagram, a sequential circuit is made up of
two major components:

• The combinational logic block, which is used to accept inputs from the external input lines
• The storage section, which is made up of the flip flops

Figure 30: Classical block diagram of a sequential circuit

Sequential logic circuits are used to implement memory systems. For example a 1-bit memory can be
implemented using an OR logic gate whose output is feedback to its input lines. (See example below):

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