So far, all of the logic circuits we have studied were basically based on the analysis and design of...
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- Slide 1
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- So far, all of the logic circuits we have studied were
basically based on the analysis and design of combinational digital
circuits. The other major aspect of a digital system is the
ANALYSIS and DESIGN of sequential digital circuits.
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- The main characteristic of combinational logic circuits is that
their output values depend on their present input values.
Sequential logic circuits differ from combinational logic circuits
because they contain memory elements so that their output values
depend on both present and past input values.
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- Combinational Circuits the logic circuits whose outputs at any
instant of time depend only on the input signals present at the
time. Sequential Circuits the logic circuits whose outputs at any
instant of time depend the present inputs as well as on the past
outputs. It consists of combinational circuits to which memory
elements are connected to form a feedback path.
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- MEMORY ELEMENTS These are the devices capable of restoring
binary information within them. The binary info stored in memory
elements at any given time defines the state of sequential circuit.
The sequential circuit receives binary information from external
inputs. These inputs together with the present state of memory
elements determine the binary value at output terminals.
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- Memory Element is a medium in which one bit of information (0
or 1) can be stored or retained until necessary, and thereafter its
contents can be replaced by a new value.
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- Sequential circuits can be Asynchronous or synchronous.
Asynchronous sequential circuits change their states and output
values whenever a change in input values occurs. Synchronous
sequential circuits change their states and output values at fixed
points of time, i.e. clock signals.
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- MASTER CLOCK GENERATOR Synchronization is achieved by timing
device called master clock generator which generates a periodic
train of clock pulses. CLOCKED SEQUENTIAL CIRCUIT Synchronous
sequential circuits that use clock pulses in the inputs of the
memory elements are called clock sequential circuit. The memory
elements used in clocked sequential circuits are called
flip-flops.
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- Block diagram of a sequential circuit NEX T STATE LOGIC MEMORY
ELEMEN TS OUTPUT LOGIC CLOCK EXTERNAL OUTPUTS EXTERNAL INPUTS
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- Latches A latch is a memory element whose excitation signals
control the state of the device. A latch has two stages set and
reset. Set stage sets the output to 1. Reset stage set the output
to 0. Flip-flops A flip-flop is a memory device that has clock
signals control the state of the device.
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- Q Q INPUTS NORMAL OUTPUT INVERTED OUTPUT The basic 1-bit
digital memory circuit is known as a flip- flop. It can have two
states either the 1 state or the 0 state. It is also known as a
bistable multivibrator. Flip-flops can be obtained by using NAND or
NOR gates.
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- RS flip-flop Master Slave JK flip-flop T flip-flop D flip-flop
JK flip-flop
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- RS Latch The RS latch is the basic memory element consists of
two cross-coupled NOR gates. It has two input signals, S set signal
and R reset signal. It also has two outputs Q and Q'; and two
states, a set state when Q = 1 and a reset state when Q = 0 (Q' =
1).
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- SRQ 00 hold 01 0 reset 10 1 set 11 unstable
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- RS Latch excitation table SRQ(t)Q(t)Q(t+1) 0000 Hold 0011 0100
Reset Q(t+1) = S(t) + R'(t)Q(t) 0110 1001 Set Q + = S+ R'Q 1011
110X Forbidden 111X
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- NAND implementation of Flip Flops Logic Diagram:
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- TRUTH TABLE SRQQ 1001 110 1 0110 1110 00 Invalid Value
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- NOR implementation of Flip Flops Logic Diagram:
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- TRUTH TABLE SRQQ 1010 0010 0101 0001 110 Invalid Value
- Slide 20
- Clocked RS Flip-flop Logic Diagram:
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- TRUTH TABLE QSRQ(t + 1) 0 000 0 011 0 101 0 11Invalid 1 001 1
010 1 101 1 11
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- Make S and R complements of each other Eliminates 1s catching
problem Can't just hold previous value (must have new value ready
every clock period) Value of D just before clock goes low is what
is stored in flip-flop Can make R-S flip-flop by adding logic to
make D = S + R' Q.
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- D Q Q' master stage slave stage P P' CLK R SQ Q'R SQ
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- With a slight modification of a J-K flip-flop, we can construct
a new flip flop called the T- FLIPFLOP. If the two inputs J and K
of a J-K flip-flop are tied together it is referred to as a
T-flip-flop. Hence a T flip-flop has only one input T and two
outputs Q and Q. The name T flip-flop actually indicates the fact
that the flip- flop has the ability to TOGGLE. It has actually only
two states TOGGLE STATE and MEMORY STATE.
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- The D flip-flop has only one input referred to as the D input,
or data input, and two output as usual Q and Q. It transfers the
data at the input after the delay of one clock pulse at the output
Q. So on some cases the input is referred to as a delay input and
the flip-flop gates the name DELAY (D) flip-flop.
- Slide 27
- A master slave flip-flop consists of two latches and an
inverter. Master-slave RS flip-flop
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- OUTPUT DECODER (combinational logic) MEMORY ELEMENTS NEXT STATE
DECODER Outputs (in) Outputs (out) Next state variables(NS)
NS=f(In,PS) Present state Variable(PS)
- Slide 29
- From the General sequential circuit model discussed in the
preceding section, sequential circuits are generally classified
into five different classes; 1.Class A Circuits 2.Class B Circuits
3.Class C Circuits 4.Class D Circuits 5.Class E Circuits
- Slide 30
- The Class A circuit is defined as a Mealy machine named after
G.H. Mealy. The basic property of a mealy machine is that the
output is a function of the present input conditions and the
present state of the circuit. The Class B and C circuits are
generally defined as a MOORE machine named after E.F. Moore. In
these types of circuits the output is strictly a function of the
present state (PS) of the circuit inputs. CLASS B CIRCUIT
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- OUTPUT DECODER MEMORY ELEMENTS NEXT STATE DECODER Outputs (in)
Outputs (out)
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- MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out)
CLASS C CIRCUIT
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- MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out)
CLASS D CIRCUIT
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- MEMORY ELEMENTS Outputs (out) CLASS E CIRCUIT Inputs (in)
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- The behavior of a sequential circuit is determined from the
inputs, the outputs, and the states of the flip-flops. Both the
outputs and the next states are a function of the inputs and the
present state. The analysis of sequential circuits consists of
obtaining a table or a diagram for the time sequence of inputs,
outputs, and internal states. Boolean expressions can be written
that describes the behavior of the sequential circuits.
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- 00 11 1001 0/0 1/0 0/0 1/01/1 0/0
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- In the state diagram, a state is represented by a circle and
the transitions between states is indicated by direct arrows
connecting the circles. The binary number inside each circle
identifies the state the circle represents. The circle arrows are
labeled with two binary numbers separated by a /. The number before
the / represents the value of the external input, which causes the
transition, and the number after the / represents the value of the
output during the present state.
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- www.meseec.ce.rit.edu/eecc341-winter2001/341-1-29- 2002.pdF
www.slideshare.net/ram_ari/sequential-logic-circuit
- Slide 41
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- 1. ____________________ differ from combinational logic
circuits because they contain memory elements so that their output
values depend on both present and past input values. a. Sequential
logic circuitsb. Logic circuits c. Sequential circuits 2. The logic
circuits whose outputs at any instant of time depend the present
inputs as well as on the past outputs. a. Sequential circuits b.
Logic circuits c. Flip-flops
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- 3. The logic circuits whose outputs at any instant of time
depend only on the input signals present at the time. a.
Combinational circuitsb. Flip-flopsc. Circuits 4. A sequential
circuit that change their states and output values whenever a
change in input values occurs. a. Asynchronous sequential circuits
b. Circuitsc. Flip-flops
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- 5. The basic 1-bit digital memory circuit is known as; a.
Flip-flops b. Circuitsc. Logic Circuits 6. The ____ is the basic
memory element consists of two cross-coupled NOR gates. a. RS
latchb. D flip-flopc. T flip-flop
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- 7.With a slight modification of a J-K flip-flop, we can
construct a new flip flop called; a. T flip-flopb. D flip-flopc. RS
latch 8. A ______ consists of two latches and an inverter. a.
master slave flip-flopb. T flip-flopc. SR flip-flop
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- 9. The Class A circuit is defined as a Mealy machine named
after; a. G.H. Mealyb. E.F. Moorec. William Moore 10. The behavior
of a sequential circuit is determined from the following except; a.
Sequenceb. inputsc. outputs