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Berkeley ELENG 42 - Logic Transients

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1Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003EECS 42 Introduction Digital ElectronicsAndrew R. NeureutherLecture # 18 Logic Transients (Handout)A) Review:Quiz 10/30 and Midterm 11/6B) Transient as Capacitor Charging C) Equivalent Resistance for MOSD) Inverter Propagation DelayE) Complementary MOS Operationhttp://inst.EECS.Berkeley.EDU/~ee42/Quiz 10/30 and Midterm 10/6Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Midterm #2 Coverage•Logic Functions and Timing Diagrams• Analysis of vanilla circuits with dependent sources• Ideal Op-Amps• Analysis of circuits using dependent sources to improve characteristics• Static analysis of logic gatesQuiz 10/30()()pSATOUTTpINDDpppSATOUTnSATOUTTnINnnnSATOUTVVVVLWkIVVVLWkI−−−−−−−−−−=−=||''2510.4PMOS1000.630.43NMOSk’(µA/V2)VOUT-SAT(V)VT(V)Minimum sized devices have W/L = 22Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Logical SynthesisGuided by DeMorgan’s TheoremDeMorgan’s Theorem :[] C B A CBA=++or[] C BA CBA=++Example of Using DeMorgan’s Theorem:Thus any sum of products expression can be immediately synthesized from NAND gates aloneCDEAB EDC BA F•=••+•=ABCDFECopyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003EFFECT OF GATE DELAYCascade of Logic GatesABCDInputs have different delays, but we ascribe a single worst-case delay τto every gateHow many “gate delays for shortest path?ANSWER : 2How many gate delays for longest path? ANSWER : 33Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003C,B,AD)BA( +)(__CB⋅BDtttttLogic stateτττ2τ2τ03ττTIMING DIAGRAMSShow transitions of variables vs timeABCNote that becomes valid two gate delays after B&C switch, because the invert function takes one delay and the NAND function a second.)(__CB⋅No change at t = 3τNote becomes valid one gate delay after B switchesB10Glitching: temporary switching to an incorrect valueCopyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003EXAMPLE CIRCUIT: INCREASED INPUT RESISTANCERIN vTEST vIN + - + - GmvIN + - vOUT = 0 RO iTEST RE vE Add resistor REAnalysis: apply iTESTand evaluate vTEST00=−−+TESTINmTESTEEEiRGiRvRvTESTININiRv=ETESTINTESTviRv+=KCLEINmINTESTTESTRRGRiv)1(++=Check for special case for R0 infiniteIntuitive Explanation:REputs RINon a node whose voltage increases in response to current in RIN.The output has been assumed to be shortedSimilar to the homework4Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Composite IOUTvs. VOUTfor CMOSVDD=5The maximum voltage is VDDVOUT(V)03IOUT(µA)2060100State 3 or VIN= 3VVOUT-SAT-DPull-Up PMOS IOUT-SAT-UPull-Down NMOS IOUT-SAT-DSolutionPD current is flat (saturated) beyond VOUT-SAT-DPU current is flat (saturated) belowVDD-VOUT-SAT-DCopyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Method for Finding VMAt VM, 1) VOUT= VIN =VM2) Both devices are in saturation3) IOUT-SAT-D= IOUT-SAT-U()()USATOUTTUINDDUUSATOUTDSATOUTTDINDDSATOUTVVVVkIVVVkI−−−−−−−−−−==−=)SubstituteVMSolve for VMExample Result: When kD= kP, VOUT-SAT-D= VOUT-SAT-Uand VTD=VTU, then VM= VDD/25Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Voltage Transfer Function for the Complementary Logic CircuitVOUT(V)VIN(V)035350State 1 for VIN= 1VState 3 for VIN= 3VState 5 for VIN = 5VVMVOUT= VINVertical section due to zero slope of IOUTvs. VOUTin the saturation region of both devices.VTUVTDVOUT-SAT-DVOUT-SAT-UPD-OffPU-OffCopyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Transient Gate Problem: Discharging and Charging Capacitance on the OutputVOUTIOUTOutputVIN-DVDDVIN-Up-type MOSTransistor(PMOS)n-type MOSTransistor(NMOS)VIN= VDD= 5VCOUT= 50 fF5V => 06Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Output Propagation Delay High to LowVOUT(0) = 5VCOUT= 50 fFIOUT-SAT-D= 100 µAVOUT(V)035IOUT(µA)2060100VIN= 5VIOUT-SAT-D= 100 µAWhen VINgoes High VOUTstarts decreases with time Assume that the necessary voltage swing to cause the next downstream gate to begin to switch is VDD/2 or 2.5V. That is the propagation delay τHLfor the output to go from high to low is the time to go from VDD= 5V to to VDD/2 =2.5VCopyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Output Propagation Delay High to Low (Cont.)When VOUT> VOUT-SAT-Dthe available current is IOUT-SAT-DVOUT(0) = 5VCOUT= 50 fFIOUT-SAT-D= 100 µAVOUT(V)035IOUT(µA)2060100VIN= 5VIOUT-SAT-D= 100 µAThe propagation delay is thusnsAVfFIVCIVCtDSATOUTDDOUTDSATOUTOUT25.11005.2502=⋅==∆=∆−−−−µFor this circuit when VOUT> VOUT-SAT-Dthe available current is constant at IOUT-SAT-D and the capacitor discharges.7Copyright 2003, Regents of University of CaliforniaLecture 18: 10/28/03 A.R. NeureutherVersion Date 10/28/03EECS 42 Intro. Digital Electronics Fall 2003Switched Equivalent Resistance ModelThe above model assumes the device is an ideal constant current source.1) This is not true below VOUT-SAT-D and leads to in accuracies.2) Combining ideal current sources in networks with series and parallel connections is problematic.OUTDDSATOUTDDOUTCRIVCt69.02==∆−−Instead define an equivalent resistance for the device by setting 0.69RDC equal to the ∆t found above()Ω==≈⋅=−−−−kAVIVIVRDSATOUTDDDSATOUTDDD5.371005434369.02µThis


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Berkeley ELENG 42 - Logic Transients

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Lecture 2

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Lecture 3

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Midterm 1

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