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ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 19: October 22, 2010 Pass Transistor Logic 1 Penn ESE370 Fall2010 -- DeHon Today • Pass Transistor Logic – Muxes – Performance – Composition – Logic – Tristates 2 Penn ESE370 Fall2010 -- DeHon Behavior • O=S*A + S*/B S A B 3 Penn ESE370 Fall2010 -- DeHon Delay • Assume R0/2 drive • 10C0 load • What else need to know? 5 – Cdiff=CSB or CDB – Assume Cdiff≈Cgate 2 5 4 Penn ESE370 Fall2010 -- DeHon Day 10 Capacitances • GS, GB, GD, SB, DB, SD 5 Penn ESE370 Fall2010 -- DeHon Day 10 Contact Capacitance • n+ contacts are formed by doping = diffusion • Depletion under contact – Contact-Body capacitance • Depletion around perimeter of contact – Also contact-Body capacitance 6 Penn ESE370 Fall2010 -- DeHon Day 10 Contact/Diffusion Capacitance • Cj – diffusion depletion • Cjsw – sidewall capacitance • LS – length of diffusion LS Cdiff C j LSW C jsw 2LS W 7 Penn ESE370 Fall2010 -- DeHon 5 Delay 2 5 8 Penn ESE370 Fall2010 -- DeHon Delay 9 Penn ESE370 Fall2010 -- DeHon CMOS Equivalent 10 Penn ESE370 Fall2010 -- DeHon CMOS Delay • O=S*A + S*/B 11 Penn ESE370 Fall2010 -- DeHon What’s different? • What’s different about the output? 12 Penn ESE370 Fall2010 -- DeHon Output ok? • Is the output usable? 13 Penn ESE370 Fall2010 -- DeHon CMOS DC Transfer Function 14 Penn ESE370 Fall2010 -- DeHon After CMOS Inverter 15 Penn ESE370 Fall2010 -- DeHon What does this do? 16 Penn ESE370 Fall2010 -- DeHon Cascade Functional? 17 Penn ESE370 Fall2010 -- DeHon Voltage Drop • Voltage drop across any number of series transistors is one Vth • Think about two series transistors as one transistor of twice the length 18 Penn ESE370 Fall2010 -- DeHon Day 9 Pinch Off • When voltage drops below VT, drops out of inversion – Occurs when: VGS-VDS< VT • Conclusion: – current cannot increase with VDS once VDS> VGS-VT – current must adjust so that VDS= VGS-VT – If current dropped to zero, then would invert and conduct again… Penn ESE370 Fall2010 -- DeHon 19 Performance? • Assume R0/2 drive • 10C0 load • Cdiff=Cgate 5 2 5 20 Penn ESE370 Fall2010 -- DeHon What does this do? A B 21 Penn ESE370 Fall2010 -- DeHon Performance • R0/2 drive • 10C0 load 5 2 5 22 Penn ESE370 Fall2010 -- DeHon Performance • R0/2 drive • 10C0 load 23 Penn ESE370 Fall2010 -- DeHon Not Isolating • Does not isolate downstream capacitive load • Stage delay now dependent on downstream stages 24 Penn ESE370 Fall2010 -- DeHon Class Ended Here 25 Penn ESE370 Fall2010 -- DeHon Power Implications • What’s the power impact of partial swing? 26 Penn ESE370 Fall2010 -- DeHon Back to Rail • How make it go to rail? 27 Penn ESE370 Fall2010 -- DeHon Transmission Gate 28 Penn ESE370 Fall2010 -- DeHon Level Restorer 29 Penn ESE370 Fall2010 -- DeHon Level Restorer 30 Penn ESE370 Fall2010 -- DeHon Level Restore • What issue arises here? 31 Penn ESE370 Fall2010 -- DeHon Level Restore • What issue arises here? 32 Penn ESE370 Fall2010 -- DeHon Tristate • Sometimes want to be able to not drive a line – Bus driven from different places – I/O port – sometimes read, sometime write 33 Penn ESE370 Fall2010 -- DeHon Tristate Driver 34 Penn ESE370 Fall2010 -- DeHon Tri-State Drivers Next week • No new assignment now – (will get new one after midterm) • Class Monday • Midterm Wednesday – No lecture – Midterm 7-9pm in this room • Class Friday 36 Penn ESE370 Fall2010 -- DeHon Midterm (Everything through today) • Restoration • Implement or identify gate / logic function • Estimate performance for circuit • Estimate/reduce energy for circuit • Size transistors in gate/netlist • Variation impact • Scaling • Ratio and pass tr circuits fair game • Synchronous/clockin g not on midterm 37 Penn ESE370 Fall2010 -- DeHon Ideas • There are other logic disciplines • We have the tools to analyze • Pass Transistor Logic – Possibly smaller, faster – Not rail-to-rail • Techniques to restore – Cascading without buffering slow – Tristate Drivers 38 Penn ESE370 Fall2010 -- DeHon