Logic Circuits and Computer Architecture

1
Logic Circuits and Computer Architecture

• Appendix A
• Digital Logic Circuits
• Part 2 Combinational and
• Sequential Circuits

2
Combinational circuits

• Each of the m outputs can be expressed as
  function of n input variables
• Truth table has
• n input columns
• m output columns
• 2n rows (all possible input combinations)

3
Binary Adder
Aa3a2a1a0
Cc4c3c2c1c0 the sum of A and B
Bb3b2b1b0
a3
a2
a0
b3
b2
b0
a1
b1
Combinational Circuit
c3
c2
c1
c0
c4
4
Seven-segment decoder
It converts a 4-bit binary-coded decimal (BCD)
value into the code required to drive a
seven-segment display
a
a b c d e f g
b
f
A B C D
Combinational Circuit
g
c
e
d
A0 B1 C1 D1
A0 B0 C0 D0
5
The truth table
6
Design Procedure

• Specification
• Write a specification for the circuit if one is
  not already available
• Formulation
• Derive a truth table or initial Boolean equations
  that define the required relationships between
  the inputs and outputs, if not in the
  specification
• Apply hierarchical design if appropriate
• Optimization
• Apply 2-level and multiple-level optimization
• Draw a logic diagram or provide a netlist for the
  resulting circuit using ANDs, ORs, and inverters

7
Design Procedure

• Technology Mapping
• Map the logic diagram or netlist to the
  implementation technology selected
• Verification
• Verify the correctness of the final design
  manually or using simulation

8
Design Example

• Specification
• BCD to Excess-3 code converter
• Transforms BCD code for the decimal digits to
  Excess-3 code for the decimal digits
• BCD code words for digits 0 through 9 4-bit
  patterns 0000 to 1001, respectively
• Excess-3 code words for digits 0 through 9 4-bit
  patterns consisting of 3 (binary 0011) added to
  each BCD code word
• Implementation
• multiple-level circuit

9
Design Example (continued)

• Formulation
• Conversion of 4-bit codes can be most easily
  formulated by a truth table
• Variables- BCD A,B,C,D
• Variables- Excess-3 W,X,Y,Z
• Dont Cares- BCD 1010 to 1111

10
Design Example (continued)

• Optimization
• 2-level usingK-maps
• W A BC BD
• X BC BD BCD
• Y CD CD
• Z D

11
Design Example (continued)

• Optimization (continued)
• Multiple-level optimization we start fromW A
  BC BD X C D BY CD Z
  G 7 10 6 0 23
• and we obtain
• W A BT
• X BT BCD where T C D
• Y CD CD
• Z D G 19

12
Design Example (continued)
13
Beginning Hierarchical Design

• To control the complexity of the function mapping
  inputs to outputs
• Decompose the function into smaller pieces called
  blocks
• Decompose each blocks function into smaller
  blocks, repeating as necessary until all blocks
  are small enough
• Any block not decomposed is called a primitive
  block
• The collection of all blocks including the
  decomposed ones is a hierarchy
• Example comparison circuit for 4-bit words
• Specification
• Input vectors A(30) and B(30) Ai Bi i-th
  element of A and B, respectively
• Output a variable E E1 if and only if AB
• Formulation
• is it convenient to derive the truth table?

14
A0
A1
A2
A3
E
B0
B1
B2
B3
Ni0 iff AiBi
15
Reusable Functions

• Whenever possible, we try to decompose a complex
  design into common, reusable function blocks
• These blocks are
• verified and well-documented
• placed in libraries for future use

16
Top-Down versus Bottom-Up

• A top-down design proceeds from an abstract,
  high-level specification to a more and more
  detailed design by decomposition and successive
  refinement
• A bottom-up design starts with detailed primitive
  blocks and combines them into larger and more
  complex functional blocks

17
Functions and Functional Blocks

• The functions considered are those found to be
  very useful in design
• Corresponding to each of the functions is a
  combinational circuit implementation called a
  functional block
• In the past, functional blocks were packaged as
  small-scale-integrated (SSI), medium-scale
  integrated (MSI), and large-scale-integrated
  (LSI) circuits.
• Today, they are often simply implemented within a
  very-large-scale-integrated (VLSI) circuit.

18
Real circuits

• 74LS00 - has four 2-input NAND gates
• Small scale integration (SSI)

19
Integrated circuits

• Scales of integration
• (Small) SSI 1-10 gates
• (Medium) MSI 10-100 gates
• (Large) LSI 100-100.000 gates
• (Very Large) VLSI gt 100.000 gates

20
Decoder (n-to-2n)

• Convert n inputs to exactly one of 2n outputs
• i.e., given an n-bit value i in input the
  decoder activates only the i-th output line

An example
decoder 2-to-4
D0
0
A0
20
D1
1
A1
2
D2
21
3
D3
21
Decoder Examples
1-to-2 decoder
A
0
A
A
D
D
D
D
1
0
0
1
2
3
A
1
0
0
1
0
0
0
D
A
A
0
1
0
0
1
0
1
0
0
1
0
0
0
1
0
D
A
A
1
1
0
0
0
1
1
1
0
(a)
D
A
A
2
1
0
2-to-4 decoder
D
A
A
3
1
0
(b)
22
A 3-to-8 decoder
23
a
d
b
c
a
b
equivalent to
c
d
a
d
b
c
a
b
equivalent to
c
d
A2
Inputs
A1
A0
3-to-8 decoder

D0
D1
D7
24
A different circuit for a 3-to-8 decoder
25
Decoder Expansion

• General procedure for building a decoder with n
  inputs and 2n outputs
• This procedure builds a decoder backward from the
  outputs
• We take 2n 2-input AND gates (output AND gates)
• The output AND gates are driven by two decoders
  with their numbers of inputs either equal or
  differing by 1
• These decoders are then designed using the same
  procedure until 1-to-2-line decoders are reached

26
Decoder Expansion - Example 1

• 3-to-8-line decoder
• Number of output ANDs 8
• Number of inputs to decoders driving output ANDs
  3
• Closest possible split to equal
• 2-to-4-line decoder
• 1-to-2-line decoder
• 2-to-4-line decoder
• Number of output ANDs 4
• Number of inputs to decoders driving output ANDs
  2
• Closest possible split to equal
• Two 1-to-2-line decoders

27
Decoder Expansion - Example 2

• 6-to-64-line decoder
• Number of output ANDs 64
• Number of inputs to decoders driving output ANDs
  6
• Closest possible split to equal
• two 3-to-8-line decoders
• 3-to-8-line decoder
• Number of output ANDs 8
• Number of inputs to decoders driving output ANDs
  3
• Closest possible split to equal
• 2-to-4-line decoder
• 1-to-2-line decoder

28
16
32
G
0
128 176
G 64 6 384
cost of a 2-level circuit
29
Decoder with Enable

• See truth table below for function
• Note use of Xs to denote both 0 and 1
• Combination containing two Xs represent four
  binary combinations

EN
A
A
D
D
D
D
1
0
0
1
2
3
0
X
X
0
0
0
0
1
0
0
1
0
0
0
1
0
1
0
1
0
0
1
1
0
0
0
1
0
1
1
1
0
0
0
1
30
E
S1
S0
31
3-to-8 decoder from two 2-to-4 decoders with
enable
D0
A0
D1
A1
D2
D3
A2
D4
D5
D6
D7
32
Combinational Logic Implementation- Decoder and
OR Gates

• Implement m functions of n variables with
• Sum-of-minterms expressions
• One n-to-2n-line decoder
• m OR gates, one for each output
• Approach 1
• Find the truth table for the functions
• Make a connection to the corresponding OR from
  the corresponding decoder output wherever a 1
  appears in the truth table
• Approach 2
• Find the minterms for each output function
• OR the minterms together

33
Decoder and OR Gates Example
Finding sum ofminterms expressions F1 ?m
(1,2,5,6,8,11,12,15)F2 ?m (1,3,4,6,8,10,13,15)
F3 ?m (2,3,4,5,8,9,14,15)
34
Exercise

• Use a decoder and or gates to build a
  combinatorial circuit with
• INPUT 3 boolean variables
• OUTPUT the number of 1s in the input (expressed
  in binary)

35
Solution Truth Table
A0 A1 A2 D1 D0
0 0 0 0 0
0 0 1 0 1
0 1 0 0 1
0 1 1 1 0
1 0 0 0 1
1 0 1 1 0
1 1 0 1 0
1 1 1 1 1
36
Solution the implementation
OR Gates
A0
A1
A2
D1
D0
37
Encoding

• Encoding - the opposite of decoding - the
  conversion of an m-bit input code to a n-bit
  output code with n m 2n such that each
  valid code word produces a unique output code
• Circuits that perform encoding are called
  encoders
• An encoder has 2n (or fewer) input lines and n
  output lines which generate the binary code
  corresponding to the input values
• Typically, an encoder converts a code containing
  exactly one bit that is 1 to a binary code
  corresponding to the position in which the 1
  appears.

38
a truth table for a 8-to-3 encoder
A2 D4 D5 D6 D7 A1 D2 D3 D6 D7 A0
D1 D3 D5 D7
39
Encoder Example

• A decimal-to-BCD encoder
• Inputs 10 bits corresponding to decimal digits 0
  through 9, (D0, , D9)
• Outputs 4 bits with BCD codes
• Function If input bit Di 1, then the output
  (A3, A2, A1, A0) is the BCD code for i
• Exercise design and realize it

40
Priority Encoder

• An encoder has two drawbacks
• If more than one input value is 1, then the
  encoder just designed does not work
• if all inputs are 0, the encoder responds as when
  D01
• Priority encorder
• Among the 1s that appear, it selects the most
  significant input position (or the least
  significant input position) containing a 1 and
  responds with the corresponding binary code for
  that position

41
Priority encoder with 4 inputs
V1 iff at least one input is 1
Xs in input part of table represent 0 or 1 thus
table entries correspond to product terms
instead of minterms
42
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43
Selecting

• Selecting of data or information is a critical
  function in digital systems and computers
• Circuits that perform selecting have
• A set of information inputs from which the
  selection is made
• A single output
• A set of control lines for making the selection
• Logic circuits that perform selecting are called
  multiplexers

44
Multiplexer (Mux) 2n-to-1

• 2n data inputs -- 1 output
• n controls, to select one of the inputs to be
  sent to the output
• Example 4-to-1 mux Truth table
• Logic symbol

S1 S0 F
0 0 D0
0 1 D1
1 0 D2
1 1 D3
45
Logic circuit for a 4-to-1 Mux
46
Example 4-to-1-line Multiplexer
Decoder
S
1
Enabling circuits
S
0
Decoder
Decoder
S
S
1
1
D
S
S
0
0
0
D
Y
Y
1
F
D
2
D
3
47
Exercise

• Consider a 2-to-1 multiplexer
• 2 data inputs D0 and D1
• 1 control input S0
• 1 data output F
• Write
• Truth table
• Logic circuits which implements it
• Extend it to deal with 4 bits at a time

48
2-to-1 mux
S F
0 D0
1 D1
D0
F
D1
S
49
Quadruple 2-to-1 mux (with enable)
50
How to use multiplexers to implement functions

• 2n-to-1 mux for a n-variable function

A B C F
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 1
1 1 1 1
51
How to use multiplexers to implement functions

• 2n-1-to-1 mux for a n-variable function

A B C F
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 1
1 1 1 1
C
FC
C
F
0
FC
1
F0
A
B
F1
52
De-multiplexer (Demux)

• 1 input -- 2n data outputs --
• n controls, to select exactly one of the outputs
  to receive the input
• Example 1-to-4 demux
• input E, controls S0 , S1
• outputs D0 , D1 , D2 , D3
• Truth table

S0 S1 D0 D1 D2 D3
0 0 E
1 0 E
0 1 E
1 1 E
53
Logic circuit for a 1-to-4 Demux
It is equal to a decoder with enable ?Sometimes
it is called decoder/demultiplexer
54
Programmable Logic Device (PLD)

• What is a PLD?
• A circuit that can be programmed after the
  manufacturing process
• Why PLD?
• A PLD can be
• made in large volumes
• programmed to implement large numbers of
  different low-volume designs

55
How to program a PLD

• Different programming technologies are used to
  control connections
• Some technologies
• Mask programming
• Fuse
• Antifuse
• Single-bit storage element
• Stored charge on a floating transistor gate
• We will see two PLDs
• ROM read only memory
• PLA Programmable Logic Array

56
Read Only Memories (ROMs)

• They are just a combinational circuits with n
  inputs and m output!
• It can be viewed as a memory with the inputs as
  addresses of data (output values)
• data are embedded into the circuit
• 2n words of m bits

A0 A1 A2 D3 D2 D1 D0
0 0 0 1 0 0 0
0 0 1 0 0 0 1
0 1 0 0 0 0 1
0 1 1 0 0 1 1
1 0 0 0 0 0 1
1 0 1 0 0 1 1
1 1 0 1 0 1 1
1 1 1 0 1 0 1
57
ROM the circuit

• A ROM is made by a decoder followed by a second
  module combining minterms to give the desired
  functions (matrix of OR gates)
• First module a decoder
• Fixed AND array with 2n outputs implementing all
  n-literal minterms.
• Second module
• Programmable OR Array with m outputs lines to
  form up to m sum of minterm expressions

58
The implementation
programmable OR array
A0
A1
A2
D0
D3
D1
D2
59
a ROM not yet programmed
programmable OR array
A0
A1
A2
D0
D3
D1
D2
60
Programmable Logic Array (PLA)

• A PLA for sum of products is made by
• a first module combining inputs to form products
  (programmable array of AND gates), followed by
• a second module combining products to give the
  desired functions (programmable array of OR
  gates)
• A PLA having a decoder as first module is a ROM

61
An example
A B C X Y
0 0 0 1 0
0 0 1 1 0
0 1 0 0 0
0 1 1 0 1
1 0 0 0 0
1 0 1 0 1
1 1 0 1 1
1 1 1 1 1

XABAB
YABBCAC
62
The PLA implementation
A
Inputs
B
AND Gates
C
AB
BC
AC
AB
Outputs
OR Gates
X
Y
63
Programmable Configurations

• ROM a fixed array of AND gates and a
  programmable array of OR gates
• PLA a programmable array of AND gates feeding a
  programmable array of OR gates

Fixed
Programmable
Programmable
Inputs
AND array
Outputs
OR array
Connections
(decoder)
ROM
Programmable
Programmable
Programmable
Programmable
Inputs
Outputs
OR array
Connections
AND array
Connections
PLA
64
Binary Addition

• Carries
• Addend-1 1 0 1 1 0
• Addend-2 1 0 1 1 1
• Sum

0
0
1
1
0
1


1
0
1
1
0
1
65
Functional Blocks Addition

• Binary addition used frequently
• Functional Blocks
• Half-Adder (HA), a 2-input bit-wise addition
  functional block,
• Full-Adder (FA), a 3-input bit-wise addition
  functional block,
• Ripple Carry Adder, a circuit performing binary
  addition, and
• Carry-Look-Ahead Adder (CLA), a hierarchical
  structure to improve performance.

66
Half-Adder

• Its just a 2-input, 2-output circuit that
  performs the following computations
• A half adder adds two bits to produce a two-bit
  sum
• The sum is expressed as a
  sum bit , S and a
  carry bit, C

67
The half adder

• Sum two binary inputs without the carry-in
• Truth table Logic Circuit

X Y S C
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1
68
Full-Adder

• A full adder is similar to a half adder, but
  includes a carry-in bit from lower stages. Like
  the half-adder, it computes a sum bit, S and a
  carry bit, C.
• For a carry-in (Z) of
  0, it is the same
  as
  the half-adder
• For a carry- in(Z) of 1

69
Full-adder

• Has to be able to deal Truth table
• with the carry-in
• Z represents the carry-in

X Y Z S C
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
70
Karnaughs maps for full adder

• S XYZXYZXYZXYZ C XY XZ YZ
• X ? Y ? Z XY XYZ XYZ
• 
  XY Z.(XYXY)
• XY Z.(X ? Y)

71
The logic circuit of a full adder
72
Binary adder

• Has to be able to deal with more bits
• An n-bit adder can be built chaining n full
  adders
• Its called ripple-carry adder

73
Ideal behaviour of circuits

• Consider an inverter (NOT gate)

74
The real behaviour

• Propagation delay time needed for a change in
  the input to affect the output (gate delay)
• Fall time time taken for the signal to fall from
  high level to low level
• Rise time time taken to rise from low to high

75
Carry Propagation

• Signals must propagate from inputs for output to
  be valid
• Carry and sum outputs of a single full-adder are
  valid c gate-delays after inputs are stable
• Value of c depends on the used technology
• In a binary adder of n bits the last carry is
  valid c?n gate-delays after inputs are stable
• For n large it may be unacceptable !

76
Carry look-ahead adder

• each carry is a function of the inputs (A and B)
• Hence, each carry can be computed by a two-level
  circuit
• Idea pre-computing all carries by means of a
  (two-level) combinatorial circuit

77
Solution

• Write a general expression for a carry
• When is a carry generated in the output?
• When does an input carry propagates to the output?

ai bi ci si ci1
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
78
General expression

• General expression for the (i1)-th carry
• ci1 aibi ci (ai bi) gi cipi
• gi aibi ? generate carry
• pi aibi ? propagate carry
• Iterate the expression for ci

79
General expression (2)

• ci1 gi pici
• gi pi(gi-1ci-1pi-1)
  gipigi-1pipi-1ci-1
• gipigi-1pipi-1(gi-2ci-2pi-2)
• gipigi-1pipi-1gi-2pipi-1pi-2ci-2
• gipigi-1pipi-1gi-2pipi-1pi-2gi-3pipi-1
  pi-2pi-3gi-4...
• It could be developed until the least significant
  input bits
• Every ci depends only on c0, pj, gj (jlti)

80
Carry expressions for a 4-bit adder

• c1 g0 p0c0
• c2 g1 p1g0 p1p0c0
• c3 g2 p2g1 p2p1g0 p2p1p0c0
• c4 g3 p3g2 p3p2g1 p3p2p1g0 p3p2p1p0c0

81
Carry Look-Ahead the architecture
a3
a2
a0
b3
b2
b0
a1
b1
generation/propagation
p3
p2
p1
p0
g3
g2
g0
g1
c0
carry look-ahead
c3
c2
c1
c0
a3
a2
a0
b3
b2
b0
a1
b1
c4
FA
FA
FA
FA
s3
s2
s0
s1
82
A practical problem

• c1 g0 p0c0
• c2 g1 p1g0 p1p0c0
• c3 g2 p2g1 p2p1g0 p2p1p0c0
• c4 g3 p3g2 p3p2g1 p3p2p1g0 p3p2p1p0c0

there is a limit due to circuit fan-in the
maximum number of inputs
83
Practical solution for n bits

• Use carry look-ahead adders for just m
  consecutive bits (4-8 is typical)
• Each of these is a stage
• Use n/m stages connected by means of the
  ripple-carry technique
• The overall delay is now (more or less) only
  c?n/m gate delays

84
A mixed solution
a15..12
b1512
a7..4
a11..8
b74
b118
a3..0
b30
c16
c12
c8
c4
CLA4
CLA4
CLA4
CLA4
c0
s15..12
s7..4
s11..8
s3..0
85
Sequential circuits

• More difficult to analyze since there is
  feedback output is fed back to input
• Need to introduce a concept of state
• Current state and next state
• Asynchronous change of state of an element is
  fed into other elements without any coordination
• Synchronous change of state of each element is
  fed into other elements only at a given instant,
  the same for all elements

86
Introduction to Sequential Circuits

• A Sequential
  circuit contains
• Storage elements
• Latches or Flip-Flops
• Combinational Logic
• Implements a multiple-output boolean
  function
• Inputs are signals from the outside
• Outputs are signals to the outside
• Other inputs, State or Present State, are signals
  from storage elements
• The remaining outputs, Next State are inputs to
  storage elements

Storage Elements
87
Initial examples

• What does this circuit do ?
• Replace inverters with NOR gates

88
SR-Latch

• Intuition
• it is a storage element that can store one bit
• Q is the bit stored into the SR-Latch
• two states
• set Q1 (Q0)
• reset Q0 (Q1)
• inputs S and R can be used to write the bit (i.e.
  to change state)

89
Analysis of SR-Latch

• Two kinds of analysis
• COMBINATIONAL
• Consider all possible configurations of S,R,Q and
  check their feasibility
• i.e. check which cofigurations are stable
• SEQUENTIAL
• Consider all possible configurations of S,R,Q at
  a generic step k and check what happens for Q at
  step k1
• i.e. consider all configurations and check if a
  stable configuration is reached

90
SR-Latch Truth TableCombinational View

• 8 possible combinations (Q NOT Q)

Q Q S R
1 0 0 0 stable
1 0 0 1 stable
1 0 1 0 unstable
1 0 1 1 unstable
0 1 0 0 stable
0 1 0 1 unstable
0 1 1 0 stable
0 1 1 1 unstable
91
SR-Latch sequential View

• Next state as a function of current state

S R Q(k) Q(k1) Q(k1)
0 0 0 0 1 reset (stable)
0 0 1 1 0 set (stable)
0 1 0 0 1 reset (stable)
0 1 1 0 1 reset (transient)
1 0 0 1 0 Set (transient)
1 0 1 1 0 Set (stable)
1 1 0 0 0 unacceptable!
1 1 1 0 0 unacceptable!
92
Q
93
First reason to avoid SR1

• When both inputs go from 1 to 0
• a race condition happens
• Both outputs are driven from 0 to 1
• Due to unpredictable physical differences one of
  the NOR gates may commute earlier from 0 to 1
• Then it will prevent the commutation of the other
  gate
• Conclusion output value is unpredictable !

94
Second reason to avoid SR1

• When both inputs go from 1 to 0
• a race condition happens
• Both outputs are driven from 0 to 1
• Both the NOR gates commute from 0 to 1 almost at
  the same time
• This drives both outputs from 1 to 0
• Both gates are again forced to commute
• This repeats again and again
• Conclusion output values oscillate !

95
Temporal evolution of SR-latch

• Time -gt
• S
• R
• Q
• Q

96
Transition table for SR-Latch
A synthetic description
S R Qn1
0 0 Qn
0 1 0
1 0 1
1 1 ---
97
Adding a clock to SR-latch

• An additional input (the clock) is used to ensure
  the latch commutes only when required
• pulses of a clock
• The latch senses S and R only when Clock1

clock cycle
98
The role of the clock

• A clock ensures commutation is propagated from
  the input to the output only when required
• But the general system clock is running
  continuously how can it be used to control a
  circuit only when needed?
• Enable Clock for the specific
  circuit
• System Clock

99
Circuit clock from system clock

• Enable Clock for the specific circuit
• System Clock

System Clock
Circuit Enable
Circuit Clock
100
A more subtle problem the latch timing problem

• while clock1,
• outputs of the latch change whenever the inputs
  change
• during the same clock cycle

Q
101
as a consequence

• In a commutation from (S1,R0) to (S0,R1) or
  from (S0,R1) to (S1,R0) the SR-latch outputs
  may be (for some time) in the unacceptable state
  where both outputs are 0
• If Q and Q are in input to a further circuit,
  this receives wrong input values, hence its
  computed output may differ from the required one

Initial state Input change Transient state Stable state
S 1 0 0 0
R 0 1 1 1
Q 1 1 0 0
Q 0 0 0 1
102
one more consequence

• Consider the following circuit and suppose that
  initially Q 0
• As long as C 1, the value of Q continues to
  change!

S
R
Desired behavior Q changes only once per clock
pulse
103
The solution master-slave Flip-Flop circuit

• it consists of two SR Latches (master and slave)
• the master (connected to circuits inputs only)
  can change its state (flip) when clock1
• The slave (connected to circuits outputs only)
  can changes its state (flop) when clock0
• Hence
• the slave reads masters outputs after they have
  stabilized
• the next circuit reads slaves outputs in the
  next clock commutation to 1, when they have
  stabilized
• In a chain of circuits this allows to control
  exactly when the (commuted) output of the i-th
  circuit acts on the input of the (i1)-th circuit

104
The solution master-slave Flip-Flop circuit
105
SR flip-flop execution example
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
0
0
106
D flip-flop a secure SR flip-flop

• Forcing R to always be NOT(S) the critical
  condition SR1 is avoided

SR flip-flop
D
S
Q
C
R
Q
107
Use of D flip-flop

• A D flip-flop is a memory cell, since it stores
  what is presented at its input
• Symbol Truth table

D Qn1
0 0
1 1
108
JK flip-flop using also SR1
109
Tabular description for JK-FF

• Input J, K State Q Output Q

J K Qn Qn1
0 0 0 0
0 1 0 0
1 0 0 1
1 1 0 1
0 0 1 1
0 1 1 0
1 0 1 1
1 1 1 0
110
Transition Tables

• Synthetic description of flip-flop dynamics

S R Qn1
0 0 Qn
0 1 0
1 0 1
1 1 ---
J K Qn1
0 0 Qn
0 1 0
1 0 1
1 1 Qn
D Qn1
0 0
1 1
111
Use of D-FF4 bit register
RE Read-Enable WE Write-Enable Pr Preset -
signals to prepare the gate writes all 1s
X0
X1
X2
X3
WE
WE.Pr
D
D
D
D
Q
Q
Q
Q
Ck
Ck
Ck
Ck
WE.Ck
RE
Y3
Y2
Y0
Y1
112
Use of D flip-flop (2)

• A D flip-flop is a delay unit, since it
  replicates at the output - one propagation delay
  later - what is presented at its input (delay
  flip-flop)
• A chain of n D flip-flops can be used to delay a
  bit value for n clock pulses

113
4 bit delay unit
RE Read-Enable SE Shift-Enable
SE
D
D
D
D
Din
Q
Q
Q
Q
Dout
Ck
Ck
Ck
Ck
SE.Ck
RE
Y3
Y2
Y0
Y1
114
4 bit shift register
X0
X1
X2
X3
WE
SE
D
D
D
D
Q
Dout
Din
Q
Q
Q
Ck
Ck
Ck
Ck
(SE.Ck) (WE.Ck)
RE
Y3
Y2
Y0
Y1
115
Register Control Signals

• WE (Write Enable) needed since many registers
  are attached to (i.e., receive data from) the
  same data bus
• SE (Shift Enable) allows a register output to
  drive next register input
• RE (Read Enable) needed since many registers are
  attached to (i.e., put data on) the same data bus

116
Counters
D0
I

• It counts the number of clock cycles in which I1
• IDEA A single JK-FF with a periodic input
  commutes its output with twice the period of its
  input
• Use a chain of JK-FF each time doubling the
  period of the input
• A counter modulo 24 is shown

D1
I.D0
D2
I.D0.D1
D3
I.D0.D1.D2
COUT
Ck
117
Temporal behaviour (1)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16



ICk
D0
118
Temporal behaviour (2)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16





ICk
D0
D1
119
Temporal behaviour (3)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16







ICk
D0
D1
D2
120
Temporal behaviour (4)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16









ICk
D0
D1
D2
D3
121
Temporal behaviour (5)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0

0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0

0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0



ICk
D0
D1
D2
D3
COUT
122
Finite State Machines (FSM)

• Called also Finite State Automata (FSA)
• Formal model useful to describe a sequential
  circuit
• Described by a table of transitions between
  states as a consequence of inputs
• If an input is true in a given state, a
  transition changes the state and may produce an
  output
• Two formal models exist
• Moore Model outputs are a function only of
  states
• Mealy Model outputs are a function of inputs and
  states

123
Graphical representation

• States are circles, transition are arrows, input
  are arrow labels, output are arrow (Mealy) or
  state (Moore) labels

Input / Output
Mealy model
Next state
Current state
Input
Next state/output
Current state/Output
Moore model
124
A very simple example of FSM

• A FSM with a binary input x, and a binary output
  z. The output z1 iff the number of 1s in the
  input sequence is even

1/0
0/0
0/1
odd
even
1/1
initial state
125
Tabular description for this FSM

• Next state as a function of current state and
  input
• Output as a function of current state and input

Current state Input Next state
even 0 even
even 1 odd
odd 0 odd
odd 1 even
Current state Input Output
even 0 1
even 1 0
odd 0 0
odd 1 1
126
Abstraction process

• FSM describes a sequential network (SN)
• SN realizes Finite State Machine
• The analysis of a SN allows to write the
  corresponding FSM
• From a FSM a SN is obtain through a synthesis
  process
• Similar to boolean functions and logical circuits
• Boolean Functions (BF) describe logical circuits
  (LC)
• LC realize Boolean Functions
• The analisys of a LC produces a BF
• LC are combinational networks (memoryless)
  synthesizing BF

127
FSA for D flip-flop

• Use Q as state descriptor (state variable)
• Use D as input
• Use Q as output
• Check for completeness

1/0
1
1/1
0/0
0
0/1
128
Its tabular description

• Output values as a function of input and current
  state values
• Next state values as a function of input and
  state value
• D flip-flop
• Output State

D Qn Qn
0 0 0
0 1 1
1 0 0
1 1 1
D Qn Qn1
0 0 0
0 1 0
1 0 1
1 1 1
129
FSA for SR flip-flop

• Use Q as state variable
• Use S and R as input
• Use Q as output
• Transitions with multiple conditions
• Unacceptable input configurations are NOT
  represented

10/0
1
00,10/1
00,01/0
0
01/1
130
FSA for JK flip-flop

• Just add condition 11 to existing transitions
• Note stability and instability of states
  according to input values

10,11/0
1
00,10/1
00,01/0
0
01,11/1
131
Synthesis of a SN from a FSA

• Identify input, output and state variables
• how many FFs are needed?
• for n states we need k FFs with 2k ? n
• label each state with a k-length bit string
• Decide which FF to use to store state values
• a D-FF is the simplest choice
• to store 0 present 0 at the input
• to store 1 present 1 at the input
• Build (and minimize) truth tables for output
  variables as a function of input and state values
• Build (and minimize) transition tables for state
  variables as a function of input and state values

132
Generic architecture of a SN
133
Example
1/0
0/1
0/0
even
odd
0
1
1/1
Q X Qnext Z
0 0 0 1
0 1 1 0
1 0 1 0
1 1 0 1
X
Z
Qnext X ? Q
Z X ? Q
134
Example 1 a given FSA
1/1
10
0/1
0/0
0/1
11
00
0/0
1/0
1/0
01
1/1
135
Example 1 variables
X
Y
Combinational circuit
An1
An
Bn1
Bn
Bn1
Bn
An1
An
Storage elements
136
Example 1 transition tables

• Transition table for output and state variables

An Bn X Y An1 Bn1
0 0 0 0 1 1
0 0 1 0 0 0
0 1 0 0 1 1
0 1 1 1 0 0
1 0 0 1 0 1
1 0 1 1 0 0
1 1 0 1 1 0
1 1 1 0 0 1
137
Example 1 minimization

• State variables Output

An1 An1 X
0 1
AB 11 1 0
01 1 0
00 1 0
10 0 0
Bn1 Bn1 X
0 1
AB 11 0 1
01 1 0
00 1 0
10 1 0
Y Y X
0 1
AB 00 0 0
01 0 1
11 1 0
10 1 1
138
Example 1 circuits
A X B X A B X
B X A X
B
A
D
D
Ck
Ck
A B X A B A X
Y
139
Example 2 specification

• Two input values are presented together
• Recognize with output 10 and 01, respectively,
  when a couple 00 or a couple 11 is presented
• Recognize with output 11 when two consecutive
  couples of identical values (00 00 or 11 11) are
  presented
• Output is 00, otherwise
• Example

INPUT 01 01 00 00 00 11 11 10 11 11 11 11
OUTPUT 00 00 10 11 10 01 11 00 01 11 01 11
140
Example 2 corresponding FSA

• Show also the initial state (double circle)

01,10/00
00
11/11
11/01
01,10/00
00/11
00/10
01,10/00
00/10
10
01
11/01
141
Example 2 variables
W
X
Y
Z
Combinational circuit
An1
An
Bn1
Bn
Bn1
Bn
An1
An
Storage elements
142
Example 2 transition tables
An Bn X Y W Z An1 Bn1
0 0 0 0 1 0 1 0
0 0 0 1 0 0 0 0
0 0 1 0 0 0 0 0
0 0 1 1 0 1 0 1
0 1 0 0 1 0 1 0
0 1 0 1 0 0 0 0
0 1 1 0 0 0 0 0
0 1 1 1 1 1 0 0
1 0 0 0 1 1 0 0
1 0 0 1 0 0 0 0
1 0 1 0 0 0 0 0
1 0 1 1 0 1 0 1
1 1 0 0 -- -- -- --
1 1 0 1 -- -- -- --
1 1 1 0 -- -- -- --
1 1 1 1 -- -- -- --
Note here unspecified inputs can be used for
minimization
143
Example 2 circuits
A X Y
A
D
Ck
B X Y
W
A X Y
Z
B X Y
B
D
Ck
144
What happens when the FSA is not complete?
1/1
10
0/1
0/0
0/1
11
00
0/0
1/0
01
1/1
145
uncomplete FSA

• Transition table for output and state variables

An Bn X Y An1 Bn1
0 0 0 0 1 1
0 0 1 - - -
0 1 0 0 1 1
0 1 1 1 0 0
1 0 0 1 0 1
1 0 1 1 0 0
1 1 0 1 1 0
1 1 1 0 0 1
Note here unspecified inputs cannot be used for
minimization
146

• State variables Output
• NOTE we are setting all unspecified values to 0

An1 An1 X
0 1
AB 11 1 0
01 1 0
00 1 --
10 0 0
Bn1 Bn1 X
0 1
AB 11 0 1
01 1 0
00 1 --
10 1 0
Y Y X
0 1
AB 00 0 --
01 0 1
11 1 0
10 1 1
147

• hence

An Bn X Y An1 Bn1
0 0 0 0 1 1
0 0 1 0 0 0
0 1 0 0 1 1
0 1 1 1 0 0
1 0 0 1 0 1
1 0 1 1 0 0
1 1 0 1 1 0
1 1 1 0 0 1
148
the corresponding FSA
1/1
10
0/1
1/0
0/0
0/1
11
00
0/0
1/0
01
1/1
149
a different choice

• State variables Output
• hence we would implement

Bn1 Bn1 X
0 1
AB 11 0 1
01 1 0
00 1 --
10 1 0
Y Y X
0 1
AB 00 0 --
01 0 1
11 1 0
10 1 1
An1 An1 X
0 1
AB 11 1 0
01 1 0
00 1 --
10 0 0
150

• hence

An Bn X Y An1 Bn1
0 0 0 0 1 1
0 0 1 1 1 0
0 1 0 0 1 1
0 1 1 1 0 0
1 0 0 1 0 1
1 0 1 1 0 0
1 1 0 1 1 0
1 1 1 0 0 1
151
the corresponding FSA
1/1
10
1/1
0/1
0/0
0/1
11
00
0/0
1/0
01
1/1