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Introduction to CMOS VLSI Design Lecture 5: Logical Effort David Harris Harvey Mudd College Spring 2004.

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1 Introduction to CMOS VLSI Design Lecture 5: Logical Effort David Harris Harvey Mudd College Spring 2004

2 CMOS VLSI Design5: Logical EffortSlide 2 Outline  Introduction  Delay in a Logic Gate  Multistage Logic Networks  Choosing the Best Number of Stages  Example  Summary

3 CMOS VLSI Design5: Logical EffortSlide 3 Introduction  Chip designers face a bewildering array of choices –What is the best circuit topology for a function? –How many stages of logic give least delay? –How wide should the transistors be?  Logical effort is a method to make these decisions –Uses a simple model of delay –Allows back-of-the-envelope calculations –Helps make rapid comparisons between alternatives –Emphasizes remarkable symmetries

4 CMOS VLSI Design5: Logical EffortSlide 4 Example  Ben Bitdiddle is the memory designer for the Motoroil 68W86, an embedded automotive processor. Help Ben design the decoder for a register file.  Decoder specifications: –16 word register file –Each word is 32 bits wide –Each bit presents load of 3 unit-sized transistors –True and complementary address inputs A[3:0] –Each input may drive 10 unit-sized transistors  Ben needs to decide: –How many stages to use? –How large should each gate be? –How fast can decoder operate?

5 CMOS VLSI Design5: Logical EffortSlide 5

6 CMOS VLSI Design5: Logical EffortSlide 6

7 CMOS VLSI Design5: Logical EffortSlide 7 Delay in a Logic Gate  Express delays in process-independent unit  3RC  12 ps in 180 nm process 40 ps in 0.6  m process

8 CMOS VLSI Design5: Logical EffortSlide 8 Delay in a Logic Gate  Express delays in process-independent unit  Delay has two components

9 CMOS VLSI Design5: Logical EffortSlide 9 Delay in a Logic Gate  Express delays in process-independent unit  Delay has two components  Effort delay f = gh (a.k.a. stage effort) –Again has two components

10 CMOS VLSI Design5: Logical EffortSlide 10 Delay in a Logic Gate  Express delays in process-independent unit  Delay has two components  Effort delay f = gh (a.k.a. stage effort) –Again has two components  g: logical effort –Measures relative ability of gate to deliver current –g  1 for inverter

11 CMOS VLSI Design5: Logical EffortSlide 11 Delay in a Logic Gate  Express delays in process-independent unit  Delay has two components  Effort delay f = gh (a.k.a. stage effort) –Again has two components  h: electrical effort = C out / C in –Ratio of output to input capacitance –Sometimes called fanout

12 CMOS VLSI Design5: Logical EffortSlide 12 Delay in a Logic Gate  Express delays in process-independent unit  Delay has two components  Parasitic delay p –Represents delay of gate driving no load –Set by internal parasitic capacitance

13 CMOS VLSI Design5: Logical EffortSlide 13 Computing Logical Effort  DEF: Logical effort is the ratio of the input capacitance of a gate to the input capacitance of an inverter delivering the same output current.  Measure from delay vs. fanout plots  Or estimate by counting transistor widths

14 CMOS VLSI Design5: Logical EffortSlide 14 Catalog of Gates Gate typeNumber of inputs 1234n Inverter1 NAND4/35/36/3(n+2)/3 NOR5/37/39/3(2n+1)/3 Tristate / mux22222 XOR, XNOR4, 46, 12, 68, 16, 16, 8  Logical effort of common gates

15 CMOS VLSI Design5: Logical EffortSlide 15 Catalog of Gates Gate typeNumber of inputs 1234n Inverter1 NAND234n NOR234n Tristate / mux24682n XOR, XNOR468  Parasitic delay of common gates –In multiples of p inv (  1)

16 CMOS VLSI Design5: Logical EffortSlide 16 Delay Plots d = f + p = gh + p

17 CMOS VLSI Design5: Logical EffortSlide 17 Delay Plots d = f + p = gh + p  What about NOR2?

18 CMOS VLSI Design5: Logical EffortSlide 18 Example: Ring Oscillator  Estimate the frequency of an N-stage ring oscillator Logical Effort: g = Electrical Effort: h = Parasitic Delay: p = Stage Delay:d = Frequency:f osc =

19 CMOS VLSI Design5: Logical EffortSlide 19 Example: Ring Oscillator  Estimate the frequency of an N-stage ring oscillator Logical Effort: g = 1 Electrical Effort: h = 1 Parasitic Delay: p = 1 Stage Delay:d = 2 Frequency:f osc = 1/(2*N*d) = 1/4N 31 stage ring oscillator in 0.6  m process has frequency of ~ 200 MHz

20 CMOS VLSI Design5: Logical EffortSlide 20 Example: FO4 Inverter  Estimate the delay of a fanout-of-4 (FO4) inverter Logical Effort: g = Electrical Effort: h = Parasitic Delay: p = Stage Delay:d =

21 CMOS VLSI Design5: Logical EffortSlide 21 Example: FO4 Inverter  Estimate the delay of a fanout-of-4 (FO4) inverter Logical Effort: g = 1 Electrical Effort: h = 4 Parasitic Delay: p = 1 Stage Delay:d = 5 The FO4 delay is about 200 ps in 0.6  m process 60 ps in a 180 nm process f/3 ns in an f  m process

22 CMOS VLSI Design5: Logical EffortSlide 22

23 CMOS VLSI Design5: Logical EffortSlide 23

24 CMOS VLSI Design5: Logical EffortSlide 24 Multistage Logic Networks  Logical effort generalizes to multistage networks  Path Logical Effort  Path Electrical Effort  Path Effort

25 CMOS VLSI Design5: Logical EffortSlide 25 Multistage Logic Networks  Logical effort generalizes to multistage networks  Path Logical Effort  Path Electrical Effort  Path Effort  Can we write F = GH?

26 CMOS VLSI Design5: Logical EffortSlide 26 Paths that Branch  No! Consider paths that branch: G = H = GH = h 1 = h 2 = F = GH?

27 CMOS VLSI Design5: Logical EffortSlide 27 Paths that Branch  No! Consider paths that branch: G = 1 H = 90 / 5 = 18 GH = 18 h 1 = (15 +15) / 5 = 6 h 2 = 90 / 15 = 6 F = g 1 g 2 h 1 h 2 = 36 = 2GH

28 CMOS VLSI Design5: Logical EffortSlide 28

29 CMOS VLSI Design5: Logical EffortSlide 29

30 CMOS VLSI Design5: Logical EffortSlide 30 Branching Effort  Introduce branching effort –Accounts for branching between stages in path  Now we compute the path effort –F = GBH Note:

31 CMOS VLSI Design5: Logical EffortSlide 31 Multistage Delays  Path Effort Delay  Path Parasitic Delay  Path Delay

32 CMOS VLSI Design5: Logical EffortSlide 32

33 CMOS VLSI Design5: Logical EffortSlide 33

34 CMOS VLSI Design5: Logical EffortSlide 34 Designing Fast Circuits  Delay is smallest when each stage bears same effort  Thus minimum delay of N stage path is  This is a key result of logical effort –Find fastest possible delay –Doesn’t require calculating gate sizes

35 CMOS VLSI Design5: Logical EffortSlide 35 Gate Sizes  How wide should the gates be for least delay?  Working backward, apply capacitance transformation to find input capacitance of each gate given load it drives.  Check work by verifying input cap spec is met.

36 CMOS VLSI Design5: Logical EffortSlide 36 Example: 3-stage path  Select gate sizes x and y for least delay from A to B

37 CMOS VLSI Design5: Logical EffortSlide 37 Example: 3-stage path Logical EffortG = Electrical EffortH = Branching EffortB = Path EffortF = Best Stage Effort Parasitic DelayP = DelayD =

38 CMOS VLSI Design5: Logical EffortSlide 38 Example: 3-stage path Logical EffortG = (4/3)*(5/3)*(5/3) = 100/27 Electrical EffortH = 45/8 Branching EffortB = 3 * 2 = 6 Path EffortF = GBH = 125 Best Stage Effort Parasitic DelayP = 2 + 3 + 2 = 7 DelayD = 3*5 + 7 = 22 = 4.4 FO4

39 CMOS VLSI Design5: Logical EffortSlide 39 Example: 3-stage path  Work backward for sizes y = x =

40 CMOS VLSI Design5: Logical EffortSlide 40 Example: 3-stage path  Work backward for sizes y = 45 * (5/3) / 5 = 15 x = (15*2) * (5/3) / 5 = 10

41 CMOS VLSI Design5: Logical EffortSlide 41 Best Number of Stages  How many stages should a path use? –Minimizing number of stages is not always fastest  Example: drive 64-bit datapath with unit inverter D =

42 CMOS VLSI Design5: Logical EffortSlide 42 Best Number of Stages  How many stages should a path use? –Minimizing number of stages is not always fastest  Example: drive 64-bit datapath with unit inverter D = NF 1/N + P = N(64) 1/N + N

43 CMOS VLSI Design5: Logical EffortSlide 43 Derivation  Consider adding inverters to end of path –How many give least delay?  Define best stage effort

44 CMOS VLSI Design5: Logical EffortSlide 44 Best Stage Effort  has no closed-form solution  Neglecting parasitics (p inv = 0), we find  = 2.718 (e)  For p inv = 1, solve numerically for  = 3.59

45 CMOS VLSI Design5: Logical EffortSlide 45 Sensitivity Analysis  How sensitive is delay to using exactly the best number of stages?  2.4 <  < 6 gives delay within 15% of optimal –We can be sloppy! –I like  = 4

46 CMOS VLSI Design5: Logical EffortSlide 46 Example, Revisited  Ben Bitdiddle is the memory designer for the Motoroil 68W86, an embedded automotive processor. Help Ben design the decoder for a register file.  Decoder specifications: –16 word register file –Each word is 32 bits wide –Each bit presents load of 3 unit-sized transistors –True and complementary address inputs A[3:0] –Each input may drive 10 unit-sized transistors  Ben needs to decide: –How many stages to use? –How large should each gate be? –How fast can decoder operate?

47 CMOS VLSI Design5: Logical EffortSlide 47 Number of Stages  Decoder effort is mainly electrical and branching Electrical Effort:H = Branching Effort:B =  If we neglect logical effort (assume G = 1) Path Effort:F = Number of Stages:N =

48 CMOS VLSI Design5: Logical EffortSlide 48 Number of Stages  Decoder effort is mainly electrical and branching Electrical Effort:H = (32*3) / 10 = 9.6 Branching Effort:B = 8  If we neglect logical effort (assume G = 1) Path Effort:F = GBH = 76.8 Number of Stages:N = log 4 F = 3.1  Try a 3-stage design

49 CMOS VLSI Design5: Logical EffortSlide 49 Gate Sizes & Delay Logical Effort:G = Path Effort:F = Stage Effort: Path Delay: Gate sizes:z = y =

50 CMOS VLSI Design5: Logical EffortSlide 50 Gate Sizes & Delay Logical Effort:G = 1 * 6/3 * 1 = 2 Path Effort:F = GBH = 154 Stage Effort: Path Delay: Gate sizes:z = 96*1/5.36 = 18 y = 18*2/5.36 = 6.7

51 CMOS VLSI Design5: Logical EffortSlide 51 Comparison  Compare many alternatives with a spreadsheet DesignNGPD NAND4-INV22529.8 NAND2-NOR2220/9430.1 INV-NAND4-INV32622.1 NAND4-INV-INV-INV42721.1 NAND2-NOR2-INV-INV420/9620.5 NAND2-INV-NAND2-INV416/9619.7 INV-NAND2-INV-NAND2-INV516/9720.4 NAND2-INV-NAND2-INV-INV-INV616/9821.6

52 CMOS VLSI Design5: Logical EffortSlide 52 Review of Definitions TermStagePath number of stages logical effort electrical effort branching effort effort effort delay parasitic delay delay

53 CMOS VLSI Design5: Logical EffortSlide 53 Method of Logical Effort 1)Compute path effort 2)Estimate best number of stages 3)Sketch path with N stages 4)Estimate least delay 5)Determine best stage effort 6)Find gate sizes

54 CMOS VLSI Design5: Logical EffortSlide 54 Limits of Logical Effort  Chicken and egg problem –Need path to compute G –But don’t know number of stages without G  Simplistic delay model –Neglects input rise time effects  Interconnect –Iteration required in designs with wire  Maximum speed only –Not minimum area/power for constrained delay

55 CMOS VLSI Design5: Logical EffortSlide 55 Summary  Logical effort is useful for thinking of delay in circuits –Numeric logical effort characterizes gates –NANDs are faster than NORs in CMOS –Paths are fastest when effort delays are ~4 –Path delay is weakly sensitive to stages, sizes –But using fewer stages doesn’t mean faster paths –Delay of path is about log 4 F FO4 inverter delays –Inverters and NAND2 best for driving large caps  Provides language for discussing fast circuits –But requires practice to master

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