Lecture 8
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CSE 260 DIGITAL LOGIC DESIGN
Combinational Circuit - 2
Design Methods Different combinational circuit design methods: Gate-level method (with logic gates) Block-level design method
Design methods make use of logic gates and useful functional blocks. These are available as Integrated Circuit (IC) chips.
Design Methods Type of IC chips (based on packing density) :
Small-scale integration (SSI): up to 12 gates Medium-scale integration (MSI): 12-99 gates Large-scale integration (LSI): 100-9999 gates Very large-scale integration (VLSI): 10,000-99,999 gates Ultra large-scale integration (ULSI): > 100,000 gates
Main objectives of circuit design: (i) reduce cost reduce number of gates (for SSI circuits) reduce IC packages (for complex circuits) (ii) increase speed (iii) design simplicity (reuse blocks where possible)
Block-Level Design Method
More complex circuits can be built using block-level method.
In general, block-level design method (as opposed to gate-level design) relies on algorithms or formulae of the circuit, which are obtained by decomposing the main problem to sub-problems recursively (until small enough to be directly solved by blocks of circuits).
4-bit Parallel Adder Consider a circuit to add two 4-bit numbers together and a carry-in, to produce a 5-bit result: X4 X3 X2 X1
C5
Y4 Y3 Y2 Y1
4-bit Parallel Adder S4 S3 S2 S1
C1 Black-box view of 4-bit parallel adder
5-bit result is sufficient because the largest result is: (1111)2+(1111)2+(1)2 = (11111)2
4-bit Parallel Adder SSI design technique should not be used. Truth table for 9 inputs very big, i.e. 29=512 entries: X4X3X2X1 0 0 0 0 0 0 0 0 0 0 0 0 ... 0 1 0 1 ... 1 1 1 1
Y4Y3Y2Y1 0 0 0 0 0 0 0 0 0 0 0 1 ... 1 1 0 1 ... 1 1 1 1
C1 0 1 0 ... 1 ... 1
Simplification very complicated.
C5 0 0 0 ... 1 ... 1
S4S3S2S1 0 0 0 0 0 0 0 1 0 0 0 1 ... 0 0 1 1 ... 1 1 1 1
4-bit Parallel Adder Alternative design possible. Addition formulae for each pair of bits (with carry in), Ci+1Si = Xi + Yi + Ci has the same function as a full adder. Ci+1 = Xi Yi + (Xi ⊕ Yi ) Ci Si = Xi ⊕ Yi ⊕ Ci
4-bit Parallel Adder Cascading 4 full adders via their carries, we get: Y4 X4
Y3 X3 C4
C5
FA
S4 Input Output
Y2 X2 C3
FA
S3
Y1 X1 C2
FA
S2
FA
S1
C1
Parallel Adders Note that carry propagated by cascading the carry from one full adder to the next.
Called Parallel Adder because inputs are presented
simultaneously (in parallel). Also, called Ripple-Carry Adder.
Examples Simple examples using 4-bit parallel adder as building blocks: 16-Bit Parallel Adder Adder cum Subtractor
4-bit Parallel Adder cum Subtractor Subtraction can be performed through addition using 2scomplement numbers.
Hence, we can design a circuit which can perform both addition and subtraction, using a parallel adder. X4 X3 X2 X1
Y4 Y3 Y2 Y1
4-bit adder cum subtractor
Result: either X+Y or X-Y
S: control signal for add/subtract
4-bit Parallel Adder cum Subtractor The control signal S=0 means add S=1 means subtract
Recall that: X-Y = X + (-Y) = X + (2’s complement of Y) = X + (1’s complement of Y) +1 X+Y = X + (Y)
4-bit Parallel Adder cum Subtractor Design requires: (i) XOR gates: Y S=0
Y
Y S=1
such that: output = Y when S=0 = Y' when S=1 (ii) S connected to carry-in.
Y'
4-bit Parallel Adder cum Subtractor Adder cum subtractor circuit: Y4 Y3 Y2 Y1 S
X4 X3 X2 X1
C
Cout
4-bit parallel adder
S4 S3 S2 S1
A 4-bit adder cum subtractor
Cin
Analysis: If S=1, then X + (1's complement of Y) +1 appears as the result. If S=0, then X+Y appears as the result.
Arithmetic Circuits: Cascading Adders Application: 6-person voting system. Use FAs and a 4-bit binary parallel adder. Each FA can sum up to 3 votes. Σ Voter 1
A
Voter 2 Voter 3
B Cin
Σ Cout
Full-adder 1 Σ Voter 4 Voter 5 Voter 6
A B Cin
Σ Cout
Full-adder 2
Σ
1 2 3 4
A
1 2 3 4
B
1 2 S 3 4
Cout
Cin
Parallel adder
3-bit Output
16-bit Parallel Adder Larger parallel adders can be built from smaller ones. Example: a 16-bit parallel adder can be constructed from four 4-bit parallel adders: X16..X13 Y16..Y13 4
C17
X12..X9 Y12..Y9
4
4-bit // adder 4
S16..S13
4
C13
X8..X5
4
4-bit // adder 4
S12..S9
Y8..Y5
4
C9
X4..X1
4
4-bit // adder 4
S8..S5
A 16-bit parallel adder
Y4..Y1
4
C5
4
4-bit // adder 4
S4..S1
C1
16-bit Parallel Adder Shortened notation for multiple lines. 4
S4 .. S1
is a shortened notation for S4 S3 S2 S1
16-bit parallel adder ripples carry from one 4-bit block to the next. Such ripple-carry circuits are “slow” because of long delays needed to propagate the carries.
Propagation Delay Every logic gate experiences some delay (though very small) in propagating signals forward.
This delay is called Gate (Propagation) Delay. Formally, it is the average transition time taken for the output signal of the gate to change in response to changes in the input signals.
Three different propagation delay times associated with a logic gate: tPHL: output changing from the High level to Low level tPLH: output changing from the Low level to High level tPD=(tPLH + tPHL)/2
(average propagation delay)
Propagation Delay Input
Output
H
Input L
Output
H L tPHL
tPLH
Propagation Delay A
B
Ideally, no
In reality, output signals
delay:
normally lag behind input signals:
1 0 1 0 1 0
C
1
Signal for A
Signal for B Signal for C time
0 1 0 1 0
Signal for A
Signal for B Signal for C time
Calculation of Circuit Delays In general, given a logic gate with delay, t. t1 t2 : tn
:
Logic Gate max (t1, t2, ..., tn ) + t
If inputs are stable at times t1,t2,..,tn, respectively; then the earliest time in which the output will be stable is: max(t1, t2, .., tn) + t
To calculate the delays of all outputs of a combinational circuit, repeat above rule for all gates.
Calculation of Circuit Delays As a simple example, consider the full adder circuit where all inputs are available at time 0. (Assume each gate has delay t.) X Y
0 0
max(0,0)+t = t
max(t,0)+t = 2t
S t
2t
max(t,2t)+t = 3t
C Z
0
where outputs S and C, experience delays of 2t and 3t, respectively.
Assignment • 4-3, 4-6, 4-17, 5-1, 5-14
Combinational Circuit - 2
Design Methods Different combinational circuit design methods: Gate-level method (with logic gates) Block-level design method
Design methods make use of logic gates and useful functional blocks. These are available as Integrated Circuit (IC) chips.
Design Methods Type of IC chips (based on packing density) :
Small-scale integration (SSI): up to 12 gates Medium-scale integration (MSI): 12-99 gates Large-scale integration (LSI): 100-9999 gates Very large-scale integration (VLSI): 10,000-99,999 gates Ultra large-scale integration (ULSI): > 100,000 gates
Main objectives of circuit design: (i) reduce cost reduce number of gates (for SSI circuits) reduce IC packages (for complex circuits) (ii) increase speed (iii) design simplicity (reuse blocks where possible)
Block-Level Design Method
More complex circuits can be built using block-level method.
In general, block-level design method (as opposed to gate-level design) relies on algorithms or formulae of the circuit, which are obtained by decomposing the main problem to sub-problems recursively (until small enough to be directly solved by blocks of circuits).
4-bit Parallel Adder Consider a circuit to add two 4-bit numbers together and a carry-in, to produce a 5-bit result: X4 X3 X2 X1
C5
Y4 Y3 Y2 Y1
4-bit Parallel Adder S4 S3 S2 S1
C1 Black-box view of 4-bit parallel adder
5-bit result is sufficient because the largest result is: (1111)2+(1111)2+(1)2 = (11111)2
4-bit Parallel Adder SSI design technique should not be used. Truth table for 9 inputs very big, i.e. 29=512 entries: X4X3X2X1 0 0 0 0 0 0 0 0 0 0 0 0 ... 0 1 0 1 ... 1 1 1 1
Y4Y3Y2Y1 0 0 0 0 0 0 0 0 0 0 0 1 ... 1 1 0 1 ... 1 1 1 1
C1 0 1 0 ... 1 ... 1
Simplification very complicated.
C5 0 0 0 ... 1 ... 1
S4S3S2S1 0 0 0 0 0 0 0 1 0 0 0 1 ... 0 0 1 1 ... 1 1 1 1
4-bit Parallel Adder Alternative design possible. Addition formulae for each pair of bits (with carry in), Ci+1Si = Xi + Yi + Ci has the same function as a full adder. Ci+1 = Xi Yi + (Xi ⊕ Yi ) Ci Si = Xi ⊕ Yi ⊕ Ci
4-bit Parallel Adder Cascading 4 full adders via their carries, we get: Y4 X4
Y3 X3 C4
C5
FA
S4 Input Output
Y2 X2 C3
FA
S3
Y1 X1 C2
FA
S2
FA
S1
C1
Parallel Adders Note that carry propagated by cascading the carry from one full adder to the next.
Called Parallel Adder because inputs are presented
simultaneously (in parallel). Also, called Ripple-Carry Adder.
Examples Simple examples using 4-bit parallel adder as building blocks: 16-Bit Parallel Adder Adder cum Subtractor
4-bit Parallel Adder cum Subtractor Subtraction can be performed through addition using 2scomplement numbers.
Hence, we can design a circuit which can perform both addition and subtraction, using a parallel adder. X4 X3 X2 X1
Y4 Y3 Y2 Y1
4-bit adder cum subtractor
Result: either X+Y or X-Y
S: control signal for add/subtract
4-bit Parallel Adder cum Subtractor The control signal S=0 means add S=1 means subtract
Recall that: X-Y = X + (-Y) = X + (2’s complement of Y) = X + (1’s complement of Y) +1 X+Y = X + (Y)
4-bit Parallel Adder cum Subtractor Design requires: (i) XOR gates: Y S=0
Y
Y S=1
such that: output = Y when S=0 = Y' when S=1 (ii) S connected to carry-in.
Y'
4-bit Parallel Adder cum Subtractor Adder cum subtractor circuit: Y4 Y3 Y2 Y1 S
X4 X3 X2 X1
C
Cout
4-bit parallel adder
S4 S3 S2 S1
A 4-bit adder cum subtractor
Cin
Analysis: If S=1, then X + (1's complement of Y) +1 appears as the result. If S=0, then X+Y appears as the result.
Arithmetic Circuits: Cascading Adders Application: 6-person voting system. Use FAs and a 4-bit binary parallel adder. Each FA can sum up to 3 votes. Σ Voter 1
A
Voter 2 Voter 3
B Cin
Σ Cout
Full-adder 1 Σ Voter 4 Voter 5 Voter 6
A B Cin
Σ Cout
Full-adder 2
Σ
1 2 3 4
A
1 2 3 4
B
1 2 S 3 4
Cout
Cin
Parallel adder
3-bit Output
16-bit Parallel Adder Larger parallel adders can be built from smaller ones. Example: a 16-bit parallel adder can be constructed from four 4-bit parallel adders: X16..X13 Y16..Y13 4
C17
X12..X9 Y12..Y9
4
4-bit // adder 4
S16..S13
4
C13
X8..X5
4
4-bit // adder 4
S12..S9
Y8..Y5
4
C9
X4..X1
4
4-bit // adder 4
S8..S5
A 16-bit parallel adder
Y4..Y1
4
C5
4
4-bit // adder 4
S4..S1
C1
16-bit Parallel Adder Shortened notation for multiple lines. 4
S4 .. S1
is a shortened notation for S4 S3 S2 S1
16-bit parallel adder ripples carry from one 4-bit block to the next. Such ripple-carry circuits are “slow” because of long delays needed to propagate the carries.
Propagation Delay Every logic gate experiences some delay (though very small) in propagating signals forward.
This delay is called Gate (Propagation) Delay. Formally, it is the average transition time taken for the output signal of the gate to change in response to changes in the input signals.
Three different propagation delay times associated with a logic gate: tPHL: output changing from the High level to Low level tPLH: output changing from the Low level to High level tPD=(tPLH + tPHL)/2
(average propagation delay)
Propagation Delay Input
Output
H
Input L
Output
H L tPHL
tPLH
Propagation Delay A
B
Ideally, no
In reality, output signals
delay:
normally lag behind input signals:
1 0 1 0 1 0
C
1
Signal for A
Signal for B Signal for C time
0 1 0 1 0
Signal for A
Signal for B Signal for C time
Calculation of Circuit Delays In general, given a logic gate with delay, t. t1 t2 : tn
:
Logic Gate max (t1, t2, ..., tn ) + t
If inputs are stable at times t1,t2,..,tn, respectively; then the earliest time in which the output will be stable is: max(t1, t2, .., tn) + t
To calculate the delays of all outputs of a combinational circuit, repeat above rule for all gates.
Calculation of Circuit Delays As a simple example, consider the full adder circuit where all inputs are available at time 0. (Assume each gate has delay t.) X Y
0 0
max(0,0)+t = t
max(t,0)+t = 2t
S t
2t
max(t,2t)+t = 3t
C Z
0
where outputs S and C, experience delays of 2t and 3t, respectively.
Assignment • 4-3, 4-6, 4-17, 5-1, 5-14
