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EE365 Adv. Digital Circuit Design Clarkson University Lecture #8 Buffers, Drivers, Encoders, MUXs & XORs
EE365Adv. Digital Circuit Design
Clarkson University
Lecture #8
Buffers, Drivers, Encoders, MUXs & XORs
Topics
• Buffers
• Drivers
• Encoders
• Multiplexers
• Exclusive OR Gates
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Three-state buffers• Output = LOW, HIGH, or Hi-Z.
• Can tie multiple outputs together, if at most one at a time is driven.
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Different flavors
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Timing considerations
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Three-state drivers
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Driver application
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Three-state transceiver
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Transceiver application
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Encoders vs. Decoders
Decoder Encoder
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Binary encoders
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Need priority in most applications
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8-input priority encoder
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Priority-encoder logic equations
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74x148 8-input priority encoder
– Active-low I/O– Enable Input– “Got Something”– Enable Output
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74x148circuit
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74x148 Truth Table
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In Class Practice Problem
Write the truth table for a 4-to-2 encoder:
• No enables
• Active High inputs and outputs
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In Class Practice Problem
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Cascading priority
encoders
• 32-inputpriority encoder
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Constant expressions
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Outputs
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Alternative formulation
• WHEN is very natural for priority function
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Multiplexers
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74x1518-input
multiplexer
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74x151 truth table
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CMOS transmission gates
• 2-input multiplexer
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Other multiplexer varieties• 2-input, 4-bit-wide
– 74x157
• 4-input, 2-bit-wide– 74x153
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In Class Practice Problem
Write the truth table for a 1-to-4 line Multiplexer:
• No enables
• Active High inputs and outputs
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In Class Practice Problem
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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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16 16-to-1 MUXs
16-to-1 mux = 2 x 74x151 8-to-1 mux + NAND gate
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4 16-bit 2-to-1 MUXs
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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2-input XOR gates• Like an OR gate, but excludes the case
where both inputs are 1.
• XNOR: complement of XOR
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XOR and XNOR symbols
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Gate-level XOR circuits• No direct realization with just a few
transistors.
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CMOS XOR with transmission gates
IF B==1 THEN Z = !A;ELSE Z = A;
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Multi-input XOR• Sum modulo 2• Parity computation
• Used to generate and check parity bits in computer systems.– Detects any single-bit error
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Parity tree
• Faster with balanced tree structure
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Next time
• Comparators
• Adders
• Multipliers
• Read-only memories (ROMs)
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