L06-4up

L06-4up - (Synchronous) Finite State Machines Great -...

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L06 – FSMs 1 6.004 – Fal 2010 9/28/10 (Synchronous) Finite State Machines Lab 2 is due tonight Great - Theory! WARD & HALSTEAD NERD KIT 6.004 Finally! Some ENGINEERING! L06 – FSMs 2 6.004 – Fal 2010 9/28/10 Flip Flop Timing - I CLK D Q D Q D CLK Q <t PD t PD : maximum propagation delay, CLK ! Q >t CD t CD : minimum contamination delay, CLK ! Q >t SETUP t SETUP : setup time guarantee that D has propagated through feedback path before master closes >t HOLD t HOLD : hold time guarantee master is closed and data is stable before allowing D to change L06 – FSMs 3 6.004 – Fal 2010 9/28/10 Flip Flop Timing - II CLK t 1 t 1 = t CD,reg1 + t CD,1 > t HOLD,reg2 1 D Q D Q CLK reg1 reg2 Questions for register-based designs: ! how much time for useful work (i.e. for combinational logic delay)? ! does it help to guarantee a minimum t CD ? How ‘bout designing registers so that t CD,reg > t HOLD,reg ? ! what happens if CLK signal doesn’t arrive at the two registers at exactly the same time (a phenomenon known as “clock skew”)? t 2 t 2 = t PD,reg1 + t PD,1 < t CLK - t SETUP,reg2 Q R1 t CD,reg1 t CD, 1 t PD, 1 t PD,reg1 Q R1 L06 – FSMs 4 6.004 – Fal 2010 9/28/10 Our New Machine Combinational Logic Current State New State Input Output Clock State Registers k k m n • Acyclic graph • Obeys static discipline • Can be exhaustively enumerated by a truth table of 2 k+m rows and k+n output columns •Engineered cycles •Works only if dynamic discipline obeyed •Remembers k bits for a total of 2 k unique combinations
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L06 – FSMs 5 6.004 – Fal 2010 9/28/10 Must Respect Timing Assumptions! Questions: Constraints on T CD for the logic? Minimum clock period? Setup, Hold times for Inputs? Combinational Logic Current State New State Input Output Clock t CD,L = ? t PD,L = 5ns t CD,R = 1ns t PD,R = 3ns t S,R = 2ns t H,R = 2ns t CD,L > 1 ns t S = t PD,L + t S,R = 7 nS t H = t H,R - t CD,L = 1 nS We know how fast it goes… But what can it do? t CD,R (1 ns) + t CD,L (?) > t H,R (2 ns) t CLK > t PD,R +t PD,L + t S,R > 10nS L06 – FSMs 6 6.004 – Fal 2010 9/28/10 A simple sequential circuit… Lets make a digital binary Combination Lock: Speci±cation: • One input ( “0” or “1”) • One output (“Unlock” signal) • UNLOCK is 1 if and only if: Last 4 inputs were the “combination”: 0110 How many registers do I need?
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L06-4up - (Synchronous) Finite State Machines Great -...

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