Topics

1. Topics • Pseudo-nMOS gates. • DCVS logic. • Domino gates. • Design-for-yield. • Gates as IP.

2. Pseudo-nMOS • Uses a p-type as a resistive pullup, n-type network for pulldowns.

3. Characteristics • Consumes static power. • Has much smaller pullup network than static gate. • Pulldown time is longer because pullup is fighting.

4. Output voltages • Logic 1 output is always at VDD. • Logic 0 output is above Vss. • VOL = 0.25 (VDD - VSS) is one plausible choice.

5. Producing output voltages • For logic 0 output, pullup and pulldown form a voltage divider. • Must choose n, p transistor sizes to create effective resistances of the required ratio. • Effective resistance of pulldown network must be comptued in worst case—series n-types means larger transistors.

6. Transistor ratio calculation • In steady state logic 0 output: • pullup is in linear region,Vds = Vout - (VDD - VSS) ; • pulldown is in saturation. • Pullup and pulldown have same current flowing through them.

7. Transistor ratio, cont’d. • Equate two currents: • Idp = Idd. • Using 0.5 mm parameters, 3.3V power supply: • Wp/Lp / Wn/Ln = 3.9.

8. DCVS logic • DCVSL = differential cascode voltage logic. • Static logic—consumes no dynamic power. • Uses latch to compute output quickly. • Requires true/complement inputs, produces true/complement outputs.

9. DCVS structure

10. DCVS operation • Exactly one of true/complement pulldown networks will complete a path to the power supply. • Pulldown network will lower output voltage, turning on other p-type, which also turns off p-type for node which is going down.

11. DCVS example

12. Precharged logic • Precharged logic uses stored charge to help evaluation. • Precharge node, selectively discharge it. • Take advantage of higher speed of n-types. • Requires multiple phases for evaluation.

13. Domino logic • Uses precharge clock to compute output in two phases: • precharge; • evaluate. • Is not a complete logic family—cannot invert.

14. Domino gate structure

15. Domino phases • Controlled by clock . • Precharge: p-type pullup precharges the storage node; inverter ensures that output goes low. • Evaluate: storage node may be pulled down, so output goes up.

16. Domino buffer • Output inverter is needed for two reasons: • make sure that outputs start low, go high so that domino output can be connected to another domino gate; • protects storage node from outside influence.

17. Domino operation

18. Domino effect Gate outputs fall in sequence: gate 1 gate 2 gate 3

19. Monotonicity • Domino gates inputs must be monotonically increasing: glitch causes storage node to discharge.

20. Output buffer • Inverting buffer isolates storage node. Storage node and inverter have correlated values.

21. Using domino logic • Can rewrite logic expression using De Morgan’s Laws: • (a + b)’ = a’b’ • (ab)’ = a’ + b’ • Add inverters to network inputs/outputs as required.

22. Domino and stored charge • Charge can be stored in source/drain connections between pulldowns. • Stored charge can be sufficient to affect precharge node. • Can be averted by precharging the internal pulldown network nodes along with the precharge node.

23. Design-for-yield • Design processes that improve chip yield in very deep submicron/nanometer technologies. • Must treat design and manufacturing as a unified processing to maximize yield in nanometer technologies.

24. Variations in manufacturing • Three types of variations: • Systematic variations can be predicted based on design and mask information plus manufacturing equipment. • Random variations include variations in parameters, etc. • Environmental variations include temperature, etc.

25. Trends in manufacturing • Larger variations in process and circuit parameters. • Higher leakage currents. • Patterning problems caused by specific combinations of geometric features. • Metal width and thickness variations. • Stress in vias.

26. Design-for-yield examples • Lithographic simulation to find yield problems not covered by standard design rules. • Extra vias added to increase the reliability of connections. • Statistical timing analysis to identify problems caused by variations in wiring.

27. Gates as IP • The standard cell library was one of the first forms of IP. • Reusable across many chips. • Portable from one process to another. • Standard cell compatibility issues: • Layout: cell size, pin placement. • Delay: driving specified load. • Power consumption.

28. Standard cell physical design • Basic cell organization is dictated by placement and routing system. • All cells are the same height. • May be one of a set of standard widths. • Pins must be placed on routing grid, usually determined by wiriing layers used.

29. Standard cell logical design • Must support a Boolean complete set of functions. • Should support enough gate types for good logic synthesis results. • Need several electrical variations of each function: • Low power. • High speed.

30. Cell verification and qualification • Cells are verified by layout extraction and circuit simulation. • Simulate a variety of process parameter combinations. • Qualification requires fabrication of cells on the target process.