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ECE 3110: Introduction to Digital Systems
Chapter 6
Combinational Logic Design Examples
Barrel ShifterSpeical purpose
Encoders/Comparators
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1
10
100
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Ex. 5.46Ex.
5.19
HW solutions
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Design examples using combinational building blocks (decoders, encoders, multiplexers, comparators, three-state devices, adders).
Barrel shifter Dual-priority encoder Cascading comparators, mode-dependent
comparator
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Barrel shifter design example n data inputs, n data outputs Control inputs specify number of
positions to rotate or shift data inputs
Example: n = 16 DIN[15:0], DOUT[15:0], S[3:0] (shift
amount) Many possible solutions, all based on
multiplexers
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Multiplexers (mux)
Select one of n sources of data to transmit on a bus.
Eg. Put between Processor’s registers and ALUA 16-bit processor where 3-bit field specifies on of 8 registers.The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux.
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MSI: 74x1518-input 1-bit multiplexer
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16-to-1 barrel shifter
16-to-1 mux = 2 x 74x151 8-to-1 mux + NAND gate
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4 16-bit 2-to-1 muxes
16-bit 2-to-1 mux = 4 x 74x157 4-bit 2-to-1 mux
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Properties of different approaches
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Dual-Priority Encoder A priority encoder identifies not
only the highest but also the second-highest-priority asserted signal among a set of 8 request inputs.
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Single-priority encoder
01001001
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01001001
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Parallel Cascading Comparators
74x85: serial cascading scheme, delay of propagating the cascading signals through a cascade of comparators.
74x682: Parallel approach, used to build very wide comparators. Build 24-bit comparator, using 3 74x682 PEQQ=EQ2.EQ1.EQ0 PGTQ=GT2+EQ2.GT1+EQ2.EQ1.GT0
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8 bit Comparator
B0
A1
B1
A2
B2
A3
A0
B3
74x85
A<BIN
A=BIN
A>BIN
A<BOUT
A=B OUT
A>BOUT
B0
A1
B1
A2
B2
A3
A0
B3
74x85
A<BIN
A=BIN
A>BIN
A<BOUT
A=B OUT
A>BOUT
B0
A1
B1
A2
B2
A3
A0
B3
B4
A5
B5
A6
B6
A7
A4
B7
+5V
A<B
A=B A>B
Most Significant bitsLeast Significant bits
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Mode-dependent Comparator
Design a c.c. whose inputs are two 8-bit unsigned binary integers(X,Y), and a control signal(MIN/MAX). The output is an 8-bit unsigned binary integer Z, such that Z=min(X,Y) if MIN/MAX=1; Z=max(X,Y) otherwise
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Next…
Sequential Logic Introduction