# VLSI Design, Fall Logical Effort. Jacob Abraham

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1 6. Logical Effort 6. Logical Effort Jacob Abraham Department of Electrical and Computer Engineering The University of Texas at Austin VLSI Design Fall 207 September 20, 207 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, 207 / 35 Review: Sizing Transistors Size the transistors in the following circuit so that it has the same drive strength (current), in the worst case, as an inverter with p-channel width of 3 and an n-channel width of 2 units. Use the smallest integer widths to achieve this ratio. Write down the size next to each transistor. ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, 207 / 35

2 6. Logical Effort 2 Review: Sizing Transistors Size the transistors in the following circuit so that it has the same drive strength (current), in the worst case, as an inverter with p-channel width of 3 and an n-channel width of 2 units. Use the smallest integer widths to achieve this ratio. Write down the size next to each transistor. ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Review: Example of Elmore Delay Calculation Calculate the Elmore delay from C to F in the circuit. The widths of the pass transistors are shown, and the inverters have minimum-sized transistors Delay = R 3 9C + R 3 5C + ( R 3 + R 3 ) 7C + 3RC = 2.33RC ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

3 6. Logical Effort 3 Review: Elmore Delay Calculation Use the Elmore delay approximation to find the worst-case rise and fall delays at output F for the following circuit. The gate sizes of the transistors are shown in the figure. Assume NO sharing of diffusion regions, and the worst-case conditions for the initial charge on a node. Input for worst-case rise delay = Worst-case rise delay = Input for worst-case fall delay = Worst-case fall delay = ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example: Delay with Different Input Sequences Find the delays for the given input transitions (gate sizes shown in figure) Assumptions: diffusion capacitance is equal to the gate capacitance, the resistance of an nmos transistor with unit width is R and the resistance of a pmos transistor with width 2 is also R, and NO sharing of diffusion regions Off-path capacitances can contribute to delay, and if a node does not need to be charged (or discharged), its capacitance can be ignored ABCD = 00 ABCD = 0 ABCD = ABCD = 0 ABCD = 00 ABCD = 0 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

4 6. Logical Effort 4 Delay with Different Input Sequence, Cont d Look at the charges on the nodes at the end of the first input of the sequence; only the capacitances of the nodes which would change with the second vector need to be considered ABCD = 00 ABCD = 0; Delay = 36RC ABCD = ABCD = 0; Delay = 6RC ABCD = 00 ABCD = 0; Delay = 43RC ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Delay Components Delay has two parts Parasitic Delay 6 or 7 RC Independent of Load Effort Delay 4h RC Proportional to load capacitance ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

5 6. Logical Effort 5 Contamination Delay Minimum (Contamination) Delay Best-case (contamination) delay can be substantially less than propagation delay Example, If both inputs fall simultaneously Important for hold time (will see later in the course) t cdr = (3 + 2h)RC ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Introduction to Logical Effort Chip designers have to 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 one 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 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

6 6. Logical Effort 6 Example Design the decoder for a register file Decoder specifications 6 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 0 unit-sized transistors Need to decide How many stages to use? How large should each gate be? How fast can decoder operate? ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Delay in a Logic Gate Express delay in a process-independent unit τ = 3RC d = d abs τ Delay has two components: d = f + p 2 ps in 80 nm process 40 ps in 0.6 µm process Effort delay, f = gh (stage effort) g: Logical Effort Measures relative ability of gate to deliver current g for inverter h: Electrical Effort=C out /C in Ratio of output to input capacitances, sometimes called fanout effort Parasitic delay, p Represents delay of gate driving no load Set by internal parasitic capacitance ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, 207 / 35

7 6. Logical Effort 7 Delay Plots d = f + p = gh + p What about NOR2? ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Computing Logical Effort 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 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

8 6. Logical Effort 8 Catalog of Gates Logical Effort of common gates Gate type Number of inputs n Inverter NAND 4/3 5/3 6/3 (n+2)/3 NOR 5/3 7/3 9/3 (2n+)/3 Tristate/Mux XOR, XNOR 4,4 6,2,6 8,6,6,8 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Catalog of Gates Parasitic delay of common gates In multiples of p inv ( ) Gate type Number of inputs n Inverter NAND n NOR n Tristate/Mux n XOR, XNOR ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

9 6. Logical Effort 9 Example: Ring Oscillator Estimate the frequency of an N-stage ring oscillator Logical Effort: g = Electrical Effort: h = Parasitic Delay: p = Stage Delay: d = 2 Frequency: f osc = /(2 N d) = /4N 3 stage ring oscillator in 0.6 µm process has frequency of 200 MHz ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example: FO4 Inverter Estimate the delay of a fanout-of-4 (FO4) inverter Logical Effort: g = Electrical Effort: h = 4 Parasitic Delay: p = Stage Delay: d = 5 The FO4 delay is about: 200 ps in a 0.6µm process 60 ps in a 80 nm process f/3 ns in a f µm process (f/3 ps in a f nm process) ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

10 6. Logical Effort 0 Example Problem A particular technology node has a FO4 delay of 9 ps. How many minimum size (2:) inverters need to be included in a ring oscillator so that the frequency is close to 7.3 GHz? FO4 delay = 5RC = 9ps Stage delay = 6RC = 3.6ps f = 2 N d = N = 2 f d = = 9.05 Number of inverters = 9 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example Problem A particular technology node has a FO4 delay of 9 ps. How many minimum size (2:) inverters need to be included in a ring oscillator so that the frequency is close to 7.3 GHz? FO4 delay = 5RC = 9ps Stage delay = 6RC = 3.6ps f = 2 N d = N = 2 f d = = 9.05 Number of inverters = 9 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

11 6. Logical Effort Example Problem A particular technology node has a FO4 delay of 9 ps. How many minimum size (2:) inverters need to be included in a ring oscillator so that the frequency is close to 7.3 GHz? FO4 delay = 5RC = 9ps Stage delay = 6RC = 3.6ps f = 2 N d = N = 2 f d = = 9.05 Number of inverters = 9 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example Problem A particular technology node has a FO4 delay of 9 ps. How many minimum size (2:) inverters need to be included in a ring oscillator so that the frequency is close to 7.3 GHz? FO4 delay = 5RC = 9ps Stage delay = 6RC = 3.6ps f = 2 N d = N = 2 f d = = 9.05 Number of inverters = 9 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

12 6. Logical Effort 2 Multistage Logic Networks Logical effort generalizes to multistage networks Path Logical Effort, G = g i Path Electrical Effort, H = C out path C in path Path Effort, F = f i = g i h i Can we write F = GH in general? ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Consider Paths that Branch G = H = 90 / 5 = 8 GH = 8 h = (5 +5)/5 = 6 h2 = 90/5 = 6 F = g g 2 h h 2 = 36 = 2GH ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

13 6. Logical Effort 3 Branching Effort and Multistage Delays Branching Effort accounts for branching between stages in path b = C on path + C off path C on path B = b i (Note : h i = BH) Now, path effort, F = GBH Multistage Delays Path Effort Delay, D F = f i Path Parasitic Delay, P = p i Path Delay, D = d i = D F + P ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Designing Fast Circuits D = d i = D F + P Delay is smallest when each stage bears same effort ˆf = g i h i = F N Thus, the minimum delay of an N-stage path is D = NF N + P This is a key result of logical effort Find fastest possible delay Doesn t require calculating gate sizes ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

14 6. Logical Effort 4 Gate Sizes How wide should the gates be for the least delay? ˆf = gh = g C out C in = C ini = g ic outi ˆf Working backward, apply capacitance transformation to find input capacitance of each gate given load it drives Check work by verifying input capacitance specification is met ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example: 3-stage Path Select gate sizes x and y for least delay from A to B ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

15 6. Logical Effort 5 Example: 3-stage Path, Cont d Logical Effort G = (4/3)*(5/3)*(5/3) = 00/27 Electrical Effort H = 45/8 Branching Effort B = 3 * 2 = 6 Path Effort F = GBH = 25 Best Stage Effort ˆf = 3 F = 5 Parasitic Delay P = = 7 Delay D = 3*5 + 7 = 22 = 4.4 FO4 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example: 3-stage Path, Cont d Work backward for sizes y = 45 * (5/3) / 5 = 5 x = (5*2) * (5/3) / 5 = 0 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

16 6. Logical Effort 6 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 N + P D = N(64) N + N ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Example of a Derivation Using the Logical Effort Equations Consider adding inverters to end of path How many give least delay? n D = NF N + p i + (N n )p inv i= D N = F N lnf N + F N + pinv = 0 Define best stage effort, ρ = F N p inv + ρ( lnρ) = 0 The above equation has no closed form solution Neglecting parasitics (setting p inv = 0), we find ρ = 2.78 (e) For p inv =, solve numerically for ρ = 3.59 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

17 6. Logical Effort 7 Sensitivity Analysis How sensitive is delay to using exactly the best number of stages? 2.4 < ρ < 6 gives delay within 5% of optimal We can be sloppy For example, use ρ = 4 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Decoder Example: Number of Stages 6 word, (32 bit) register file Each bit presents load of 3 unit-sized transistors True and complementary address inputs A[3:0] Each input may drive 0 unit-sized transistors Find: number of stages, sizes of gates, speed Decoder effort is mainly electrical and branching Electrical Effort: H = (32*3)/0 = 9.6 Branching Effort: B = 8 If we neglect logical effort (assume G = ) Path Effort: F = GBH = 76.8 Number of Stages: N = log 4 F = 3. Try a 3-stage design ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

18 6. Logical Effort 8 Decoder: Gate Sizes and Delay Logical Effort: G = * 6/3 * = 2 Path Effort: F = GBH = 54 Stage Effort: ˆf = F 3 = 5.36 Path Delay: D = 3 ˆf = 22. Gate sizes: z = 96*/5.36 = 8 Gate sizes: y = 8*2/5.36 = 6.7 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Decoder: Comparison Compare many alternatives with a spreadsheet Design N G P D NAND4-INV NAND2-NOR2 2 20/ INV-NAND4-INV NAND4-INV-INV-INV NAND2-NOR2-INV-INV 4 20/ NAND2-INV-NAND2-INV 4 6/ INV-NAND2-INV-NAND2-INV 5 6/ NAND2-INV-NAND2-INV-INV-INV 6 6/ ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

19 6. Logical Effort 9 Review of Definitions Term Stage Path Number of stages N Logical effort g G = g i Electrical effort Branching effort h = Cout C in b = C on path+c off path C on path H = C out path C in path B = b i Effort f = gh F = GBH Effort delay f D F = f i Parasitic delay p P = p i Delay d = f + p D = d i = D F + P ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35 Method of Logical Effort. Compute path effort F = GBH 2. Estimate best number of stages N = log 4 F 3. Sketch path with N stages 4. Estimate least delay D = NF N + P 5. Determine best stage effort ˆf = F N 6. Find gate sizes C in = g ic out f 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 significant wires Maximum speed only Not minimum area/power for constrained delay ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

20 6. Logical Effort 20 Summary of Logical Effort 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 ECE Department, University of Texas at Austin Lecture 6. Logical Effort Jacob Abraham, September 20, / 35

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