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L07-CombWrapup

# L07-CombWrapup - Fall 2009 ECE 18-240 Fundamentals of...

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Fall 2009 ECE 18-240 Fundamentals of Computer Engineering LEC 7: Combinational Logic Wrapup: Mixed-Logic Conventions Don Thomas & William Nace Electrical & Computer Engineering Carnegie Mellon University LEC 7 Fall-09 18-240 LEC7 — 18-240: Where are we...? ! 1 Handout: Lec7 " HW2 is out now on Bb. " Lab1 continues this week, part 1B this week. ! Lab 2A starts next week (Tues). Note: Exam 1 in 2 weeks; covers up to Lec7

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Fall-09 18-240 LEC7 — Combinational Logic Wrap- ! What you know " Lots of ways to describe a boolean circuit " Techniques and strategies to simplify boolean circuits " Use of Verilog to describe and synthesize circuits " Some larger, multi-gate, combinational components ! What you don’t know " Mixed-logic notation ! Positives / negative logic ideas and notations ! The importance of picking good signal names ! Ways not to get confused by bars and bubbles… " High impedance drivers ! What you would like to forget " 1 st midterm is Oct 1 (2 weeks from today) ! Covers L00 - L07 Combinational Logic Combinational Logic Fall-09 18-240 LEC7 — Blobs of Combinational Logic ! Characteristics " outputs a function of inputs only " can be described by assign or always ! we’re leaving gates behind mostly ! can describe some big blobs of logic here " each assign or always uniquely drives a signal ! no one else drives it ! OK, we told a little fib! Others can, but only in very specific situations assign f = … blah blah blah ; always @* begin if (blah blah) f = … blah blah blah ; end
Fall-09 18-240 LEC7 — Tri-State Drivers ! Before, we said " “don’t connect two gate outputs together” and " “a logic signal can only be driven by one always (or assign) block ! Tri-state drivers " Tri-state driver (also called tri-state “buffer”) allows multiple gate outputs to drive a single wire, just not at the same time " Output can be 0, 1, or “ z ”; resulting value on wire can be 0, 1, x, or z " z ” stands for high-impedance … “ electrically disconnected A B sel’ bus A C en’ A en’ C 0 0 0 1 0 1 0 1 z 1 1 z A B sel’ bus A B 0 A A B 1 B sort of like a mux …it is! … sort of. tri-state driver symbol ensures only one driver at a time! Bus — “shared connection” Fall-09 18-240 LEC7 — Tri-State Example ! How to think about tri-state drivers " In this example, A is “driven” onto bus. B is “tristated”, disconnected ! Is it really just like a mux? " It has the function of a mux — one of n things is selected. A B sel’= 0 bus A B sel’= 0 bus = A The impedance (resistance) is sooo high, that there is no connection.

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Fall-09 18-240 LEC7 — But you can pull it apart ! It’s a distributed mux… A B sel’= 0 bus A B sel’= 0 lots of distance lots of distance bus Fall-09 18-240 LEC7 — Mux and Tri-state Differences ! Typical mux usage " A and B are nearby ! Typical tri-state usage " A and B are not nearby.
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