Presentation on theme: "ECE 3110: Introduction to Digital Systems Chapter 5 Combinational Logic Design Practices Encoders Three-state devices Multiplexers."— Presentation transcript:
ECE 3110: Introduction to Digital Systems Chapter 5 Combinational Logic Design Practices Encoders Three-state devices Multiplexers
2 74x138/139 decoders
3 Encoders Multiple-input/multiple-output device. Perfoms the inverse function of a Decoder. Outputs ( m ) are less than inputs ( n ). Converts input code words into output code words. input code output code ENCODER
14 Three State Buffers/Drivers A buffer/inverter with enable input Buffer Buffer Inverter Inverter Actice High Enable Actice Low Enable Active High Enable Actice Low Enable The device behaves like an ordinary buffer/inverter when the enable input is asserted. The ouput is floating ( High Impedance, Hi-Z ) when the enable input is deasserted ( The input is isolated from the output, behaves as if it did not exist) Application: Controlling the access of a single line/bus by multiple devices
15 Three-state buffers Output = LOW, HIGH, or Hi-Z. Can tie multiple outputs together, if at most one at a time is driven.
16 8 sources share a three-state party line
17 Timing considerations
18 Standard SSI/MSI 3-state buffers SSI: 74x125, 74x126 (independent enable inputs) MSI: 74x541 and varieties such as 74x540, 74x240, 74x241
21 Pairs of 3-state buffers connected in opposite directions between each pair of pins, so data can be transferred in either direction. DIR determines the direction of transfer (A-->B or B-->A) Three-state transceiver
22 Transceiver application Bidirectional buses
23 Multiplexers (mux) Select one of n sources of data to transmit on a bus. Eg. Put between Processor’s registers and ALU A 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.
28 Demultiplexers A mux is used to select one of n sources of data to transmit on a bus. A demultiplexer can be used to route the bus data to one of m destinations. Just the inverse of a mux. A binary decoder with an enable input can be used as a demux. Eg. 74x139 can be used as a 2-bit, 4- output demux.
29 Next… X-OR gates and Parity circuits Comparators Adders, subtractors, ALUs Reading Wakerly CH-5.8-5.11