lecture5_comb

lecture5_comb - Combinational Circuits Z. Jerry Shi...

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1 Combinational Circuits Z. Jerry Shi Computer Science and Engineering University of Connecticut Thank John Wakerly for providing his slides and figures. Three-state buffers • Output = LOW, HIGH, or Hi-Z – High-impedance – Wire can be considered disconnected • Can tie multiple outputs together if at most one at a time is enabled
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2 CMOS three-state buffer Different flavors
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3 Three-state drivers Driver application
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4 Three-state transceiver Transceiver application
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5 CMOS NAND gate Open-drain CMOS
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6 Open-drain CMOS gate driving a load Driving an LED
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7 Wired-AND Decoders • General decoder structure • Typically n inputs, 2 n outputs – 2-to-4, 3-to-8, 4-to-16, etc.
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8 Decoder applications • Generate enable signals (enable one out of many) – Microprocessor memory systems • Selecting different banks of memory – Microprocessor input/output systems • Selecting different devices – Microprocessor instruction decoding • Enabling different functional units – Memory chips • Enabling different rows of memory depending on address – Lots of other applications Binary 2-to-4 decoder Note “x” (don’t care) notation.
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9 2-to-4-decoder logic diagram MSI 2-to-4 decoder • Input buffering (less load) • NAND gates (faster)
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10 Complete 74x139 Decoder Decoder symbol
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11 More decoder symbols 3-to-8 decoder
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12 Decoder cascading 4-to-16 decoder More cascading 5-to-32 decoder
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13 Design with decoders • Build a 4-16 decoder with 2-4 decoders • Implement 2-variable functions with 2-4 decoders • How about 3-variable functions? • Select one for four signals Sharing a wire
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14 Timing considerations [2-0] Hi-Z state Multiple sources sharing BUS
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15 Switching equivalent Multiplexers
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16 2-1 multiplexer 4-1 Multiplexer
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17 74x153 • 4-input, 2-bit-wide 74x157: 2-input 4-bit multiplexer
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18 74x151 8-input multiplexer 74x151 truth table
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19 Implement any 3-variable with 74-x151 Can we do it with 2-1 a multiplexer?
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20 CMOS transmission gates and multiplexer • A transmission gate works like a switch • Can be used to implement muxes • Be careful when using transmission gates
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lecture5_comb - Combinational Circuits Z. Jerry Shi...

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