Chapter 11_2 (10_2 edition 8)

Sequential Circuits etc.

Read-Only Memory (ROM)

• Memory that is implemented with combinational circuits– Combinational circuits are often referred to as

“memoryless” circuits because their output depends only on their current input and no history of prior inputs is retained

Read-Only Memory (ROM)

• Memory unit that performs only the read operation– Binary information stored in a ROM is permanent

and is created during the fabrication process– A given input to the ROM (address lines) always

produces the same output (data lines)– Because the outputs are a function only of the

present inputs, ROM is a combinational circuit

Sequential Circuit

Current output depends not only

on the current input, but also on the past history

of inputs

Makes use of combinational

circuits

Sequential

Circuit

Sequential circuits• The flip-flop is a bi-stable device. • It exists of two states and in the absence

of input, remains in that state. • Thus the flip-flop can function as a 1-bit

memory.• The flip-flop has two outputs, which are

always complements of each other. • These are generally labelled as Q and Q

The S-R Latch

SR Characteristic table

+NOR S-R Latch Timing Diagram

Clocked S-R Flip-flop

•

D-Flip-flop

J-K Flip Flop

8-Bit parallel register

5-Bit shift register

CountersCounters are asynchronous or synchronous• Asynchronous

slowoutput of one flip-flop triggers change of status in next flip-flop

• Synchronous all flip-flops change state at same timefaster – used in CPUs

Irvine, Kip R. Assembly Language for Intel-Based Computers, 2003.

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