L09-FSM1StTransDia - Fall 2009 ECE 18-240 Fundamentals of...

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Fall 2009 ECE 18-240 Fundamentals of Computer Engineering LEC 9: Sequential Logic and FSMs Don Thomas & William Nace Electrical & Computer Engineering Carnegie Mellon University LEC 9 Midterm 1 Thursday next week Up through L7 SPR09 18-240 LEC9 — 18-240: Where are we...? ! 1 Handout: Lec9 " HW2 due in your recit section. Short QUIZ in your recit, too " Lab2 starts this week, part 2A this week. 2
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Fall-09 18-240 Lec9 — Sequential Logic and FSMs ! What you (probably) know " There are two types of digital circuits: combinational and sequential " You’ve probably seen state transition diagrams before ! What you probably don’t know " How to draw state transition diagram of a sequential system, how to use Verilog, and complete a sequential logic design, where are the trade-offs ! This lecture " What’s a sequential circuit? What’s a state transition diagram? " What’s a D flip-flop? 3 Fall-09 18-240 Lec9 — Sequential Circuits ! There are two broad classes of digital circuits ! Combinational " You know, we just spent several weeks on them " Outputs are a function (solely) of the current inputs ! Sequential " These circuits have memory — a new class of components " Outputs are a function not only of current inputs, but also of (potentially all) previous inputs " The notion of all previous inputs is captured in memory devices ! The most important style of sequential design uses sequential and combinational elements such that the design steps through a sequence of actions or situations called states " aka State machines, finite state machines, FSMs, sequential machines " For the most part, we’ll call them FSMs " BTW, “sequential elements” are different — they’re the memory elements themselves, like latches and flip flops 4
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Fall-09 18-240 Lec9 — What is a sequential element? ! D Flip Flops " A one-bit storage devices, sometimes called a sequential element " It “flip flops” between two states. ! What are the ports? " D is the data input — the value to be remembered " C is the clock input. The triangle indicates that it’s edge triggered (we will explain that one .... ) " Q is the output (and you get Q’ for free if you want it) ! History Lesson " T flip-flops (value “Toggles”) " SR flip-flop (“Set or Reset”) " JK (combines SR and T) One of many. The most simple and common Edge triggered D-FF symbol Q D C clock Q and Q’ data Q 5 Nobody except Digital Design Textbook authors and some professors care anymore Clk D Q + 0 X Q 1 X Q 0 0 1 1 Fall-09 18-240 Lec9 — D Flip-flop: How does it work? edge triggered F/F symbol Timing Diagram: D CLK Q time Q D C clock usually can get Q’ for free data 6 Q’
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Fall-09 18-240 Lec9 — Edge Triggered Clock transition determines when input is acquired The point in time where clock changes value is called the “edge”. This is a “positive edge” since clock is rising 0 # 1.
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