1. Three-state devices Multiplexers
ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices
Three-state devices
Multiplexers

2. Three State Buffers/Drivers
A buffer/inverter with enable input
Buffer Buffer Inverter Inverter Active High Enable Active Low Enable Active High Enable Active Low Enable
The device behaves like an ordinary buffer/inverter when the enable input is asserted.
The output 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

3. Three-state buffers Output = LOW, HIGH, or Hi-Z.
Can tie multiple outputs together, if at most one at a time is driven.

4. 8 sources share a three-state party line

5. Timing considerations

6. Standard SSI/MSI 3-state buffers
SSI: 74x125, 74x126 (independent enable inputs)
MSI: 74x541 and varieties such as 74x540, 74x240, 74x241

7. Octal non-inverting 3-state buffer
Hysteresis

8. Driver application

9. Three-state transceiver
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)

10. Transceiver application
Bidirectional buses

11. Multiplexers (Mux)
E.g. Put between Processor’s registers and ALU
A 16-bit processor where 3-bit field specifies one of 8 registers.
The 3-bit field is connected to the select inputs of an 8-input, 16-bit Mux.
Select one of n sources of data to transmit on a bus.

12. MSI: 74x151 8-input 1-bit multiplexer

13. 74x151 truth table

14. Other multiplexer varieties
2-input, 4-bit-wide
74x157
4-input, 2-bit-wide
74x153

15. Expanding Multiplexers
32-to-1 Mux

16. 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. E.g. 74x139 can be used as a 2-bit, 4-output Demux.

17. Homework #8 (Due:     04/09/2007 )
Notes: On all timing calculation problems, describe the circuit path used and show each number in the calculation.
Eg. Twct=T1+T2
T1=TpHL (specify inputs that are causing the transition, i.e. which row. Also show if you picked max or typical Tp values)

18. Next… XOR gates and Parity circuits Comparators
Reading Wakerly CH