Tristate Outputs in Digital Systems
Learn about tristate outputs in digital systems where outputs can be 1, 0, or high-impedance (hi-Z), understand the significance of tristates, how to implement logic with tristates, and examples of using tristates effectively in digital circuits.
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Presentation Transcript
ECE 352 Digital System Fundamentals Tristates Tristates 1 1
Three-State Outputs Three-state ( tristate ) outputs have three possible conditions: 1, 0, high-impedance (hi-Z) The high impedance condition is denoted as Z Z is the lack of a 1 or 0 Output terminal is disconnected from both power and ground High impedance means an open circuit -- it is not connected Cannot drive a Z value onto a wire Often used to allow different drivers to share a wire/bus at different times Tristates Be careful of contention! Contention occurs when more than one gate drives a wire at the same time Only one driver can be active at a time! 2 2
Tristate Buffer Passes input if enabled, otherwise open circuit (disconnected) Truth Table: A EN 0 0 0 1 1 0 1 1 F Z 0 Z 1 Tristates Gate Symbol: EN A F Remember that Z is not a logic value! Z means DISCONNECTED 3 3
Using Tristates Example: Can use tristates to select data inputs in a multiplexer DEC 2:4 S1 S0 1 3 2 1 0 0 D0 Y Tristates D1 D2 D3 4 4
Implement Logic With Tristates Remember unless told otherwise, a circuit must output 1 or 0 for every input combination Easiest way to ensure this while avoiding contention is to use tristates enabled by a DECODER Otherwise, use tristates in pairs Tristates Examples: F = A K = G M E = C + D 0 G 1 A 1 F M 0 C D K E Notice that each of these tristate pairs is a 2:1 multiplexer 5 5
More Complicated Functions Methodology work backwards from the output Choose one variable as the last select Use Boolean algebra to recast the function in terms of sub-functions for the select being true vs. false Repeat on each sub-function Example: Y = A B C + B C = B( AC) + B( C) F = AC = A(C) + A(0) G = C = C(1) + C(0) Tristates = B(F) + B(G) 0 A 0 A C C 0 F F F B B Y Y Y Y B B G G G C 1 6 6
What Not To Do B A B B A 1 A C What if B = 0? What if B = 0? What if A = 0? What if A = 0? What if B = 0? Tristates A A C D 1 1 B B What if A and B are both 0? What if A and B are both 1? 7 7
What To Do When building logic functions using tristates: Make sure every gate input is driven by a valid logic signal (0 or 1) at all times Make sure the output is a valid logic signal (0 or 1) for all possible input combinations Make sure a wire is never driven by two or more tristate outputs at the same time Tristates with connected outputs are normally enabled by a decoder or similar logic Make sure a wire is never connected to only one tristate output Tristates 8 8
ECE 352 Digital System Fundamentals Tristates Tristates 9 9
