Transcript ppt
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 7: September 22, 2010 Delay and RC Response 1 Penn ESE370 Fall2010 -- DeHon Delay is RC Charging 2 Penn ESE370 Fall2010 -- DeHon Delay is RC Charging • Understand switch state • Break into stages • For each stage – Understand Rdrive – Understand Cload 3 Penn ESE370 Fall2010 -- DeHon Today • • • • • • RC Charging RC Step Response What is the C? What is the R? Measuring Delay Delay model of gate 4 Penn ESE370 Fall2010 -- DeHon 90% Rise Time? 5 Penn ESE370 Fall2010 -- DeHon What does response look like? 6 Penn ESE370 Fall2010 -- DeHon What does look like? V (t) 1 e t /(RC ) V 7 Penn ESE370 Fall2010 -- DeHon Shape of Curve x e-x 1-e-x 0 0.1 1 2 2.3 V (t) 1 e Penn ESE370 Fall2010 -- DeHon t /(RC ) V 8 Shape of Curve x e-x 1-e-x 0 0.1 1 2 2.3 1 0.9 1/e = 0.37 1/e2 = 0.14 0.1 0 0.1 0.66 0.86 0.9 V (t) 1 e Penn ESE370 Fall2010 -- DeHon t /(RC ) V 9 What is C? 10 Penn ESE370 Fall2010 -- DeHon Capacitance • • • • Wire MOSFET gate Logical Gate Fanout -- Total gate load 11 Penn ESE370 Fall2010 -- DeHon Wire Capacitance 12 Penn ESE370 Fall2010 -- DeHon Wire Capacitance A C r 0 d Penn ESE370 Fall2010 -- DeHon 13 There are always Rs and Cs • Modeling vs. discrete components • Dominant effects – Rbig + Rsmall ≈ Rbig – Cbig || Csmall ≈ Cbig 14 Penn ESE370 Fall2010 -- DeHon First Order Model • Switch – Loads input capacitively • As dig in, understand: – Origin of capacitance – How can we engineer – Tradeoffs 15 Penn ESE370 Fall2010 -- DeHon Logic Gate Input Capacitance • Capacitance on – A input? – B input? 16 Penn ESE370 Fall2010 -- DeHon Fanout in Circuit • Output routed to many gate inputs 17 Penn ESE370 Fall2010 -- DeHon Fanout in Circuit • Maximum fanout? • Second? • Min? 18 Penn ESE370 Fall2010 -- DeHon Lumped Capacitive Load Cload C gi i fanout C wi iwires 19 Penn ESE370 Fall2010 -- DeHon What is R? 20 Penn ESE370 Fall2010 -- DeHon Resistance • Wire resistance – Supply to transistor source – Transistor output gate it is driving • Transistor equivalent resistance 21 Penn ESE370 Fall2010 -- DeHon Wire Resistances 22 Penn ESE370 Fall2010 -- DeHon First Order Model • Switch – Resistive driver • As dig in, understand: – More sophisticated view – How can we engineer – Tradeoffs 23 Penn ESE370 Fall2010 -- DeHon Equivalent Resistance • What resistances might transistors contribute? – How many cases? 24 Penn ESE370 Fall2010 -- DeHon Lumped Resistive Source Rdrive Rtrnet R wi R pwr iwires Rtrnet = parallel and series combination of Rtr 25 Penn ESE370 Fall2010 -- DeHon Measuring Delay 26 Penn ESE370 Fall2010 -- DeHon Measuring Gate Delay • Next stage starts to switch before first finishes • Measure 50%--50% 67ps 80ps 27 Penn ESE370 Fall2010 -- DeHon Characterizing Gate/Technology • Delay measure will be – Function of load on gate – Function of input rise time • Which, in turn, may be a function of input loading 28 Penn ESE370 Fall2010 -- DeHon Delay vs. Risetime 1ps rise 10ps delay 100ps rise 20ps delay 29 Penn ESE370 Fall2010 -- DeHon Characterizing Gate/Technology • Delay measure will be – Function of load on gate – Function of input rise time • Which, in turn, may be a function of input loading • Want to understand typical – At least comparable 30 Penn ESE370 Fall2010 -- DeHon Measuring/Characterizing • Drive with a gate – Not an ideal source • Measure loaded gate – Typical loading – FO4 31 Penn ESE370 Fall2010 -- DeHon HW2 Recommendation 32 Penn ESE370 Fall2010 -- DeHon Rise/Fall • Rise and Fall time may differ – Why? 33 Penn ESE370 Fall2010 -- DeHon Delay Model of Gate 34 Penn ESE370 Fall2010 -- DeHon Sample Gate 35 Penn ESE370 Fall2010 -- DeHon Sample Gate • Internal stages have delay • External depend on load • Assume (guarantee) all inputs same load t gate A B fanout Penn ESE370 Fall2010 -- DeHon 36 Sample Gate • What is A? • B? t gate A B fanout Penn ESE370 Fall2010 -- DeHon 37 Admin • HW1 grades posted • HW2 due Friday • Here on Friday for lecture 38 Penn ESE370 Fall2010 -- DeHon Delay is RC Charging 39 Penn ESE370 Fall2010 -- DeHon