1. Day 16: October 12, 2012 Performance: Gates
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems
Day 16: October 12, 2012
Performance: Gates
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2. First Order Delay t = R0C0 R0 = Resistance of minimum size NMOS device
Previously:
First Order Delay
R0 = Resistance of minimum size NMOS device
C0 = gate capacitance of minimum size NMOS device
Rdrive = R0/Wn Cg = WC0
Technology independent relative delay
t = R0C0
Large fanout – drive in stages
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3. Today Delay in Gates Data Dependent Delay Large Fanin
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4. Gates
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5. Data Dependent Delay Resistance depends on input values
delay depends on input data
t-delays assuming minsize?
assume 2C0 load
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6. How Size
How size to equalize worst-case rise/fall times for Rdrive=R0/2?
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7. How Size How size for equal rise/fall for Rdrive=R0/2?
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8. Input Load Input capacitance per input in each case?
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9. Observe
Ratio of Input Load Capacitance to Output Drive Strength: CILoad/Ids
Differs with gate function
Some gates give more drive per capacitive load we pay
When Ids differ at same W
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10. How Size Size equalize rise/fall times Rdrive=R0/2?
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11. Increasing Fanin
What happens to input capacitance as fanin (k) increases
Keeping output drive the same
E.g. Rdrive=R0/2
k-input nand gate has what input capacitance?
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12. Fanin Conclude: gates slow down with fanin
Less drive per input capacitance
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13. Which is Fastest? nand32 nand4-inv-nand4-inv-nand2 (nand2-inv)4-nand2
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14. Delay of each implementation?
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15. Take Away?
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16. Lesson Large gates are slow / inefficient
High capacitive load / drive strength
Small gates can be inefficient
Need many stages
Staging over moderate size gates minimizes delay
Exact size will be technology dependent
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17. Admin HW5 Problem 4 – work sizing and delay parts (a and c) now
Energy next time
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18. Ideas First order reason in t=R0C0 units
Gates have different efficiencies
Drive strength per unit input capacitance
Without velocity saturation
Reason to prefer nand over nor
With velocity saturation (short term),
nands and nors are similar efficiency
Large fanin and fanout slow gates
Decompose into stages
…but not too much
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