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Gate Design

Gate Design. Static complementary logic gate structures. Switch logic. Other Gate issues. Static complementary gates. Complementary have complementary pullup (p-type) and pulldown (n-type) networks. Static do not rely on stored charge. Advantage of Static complementary gates

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Gate Design

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  1. Gate Design • Static complementary logic gate structures. • Switch logic. • Other Gate issues

  2. Static complementary gates • Complementary • have complementary pullup (p-type) and pulldown (n-type) networks. • Static • do not rely on stored charge. • Advantage of Static complementary gates • Simple, effective, reliable; hence ubiquitous.

  3. Static complementary gate structure Pullup and pulldown networks: VDD pullup Network(P type) out inputs pulldown Network(N type) VSS

  4. If the input voltage is '1' (VCC) P-type transistor on top is nonconducting N-type transistor is conducting and provides a path from GND to the output. The output therefore is '0'. Inverter + out a

  5. NAND gate + out b a

  6. NOR gate + b a out

  7. AOI/OAI gates • AOI • and/or/invert • OAI • or/and/invert. • Why ? • Implement larger functions. • Pullup and pulldown networks are compact: • smaller area, higher speed than NAND/NOR network equivalents. • AOI312 • and 3 inputs, and 1 input (dummy), and 2 inputs; or together these terms; then invert.

  8. AOI example out = [ab+c]’: invert symbol circuit or and

  9. Pullup/pulldown network design • Pullup and pulldown • Networks are duals. • To design one gate • First design one network • Then compute dual to get other network. • Example: • design network which • pulls down when output should be 0 • then find dual to get pullup network

  10. a a dummy c b b c dummy Dual network construction

  11. Switch logic • Can implement Boolean formulas • as networks of switches. • Can build switches • from MOS transistors—transmission gates. • Transmission gates • do not amplify but have smaller layouts.

  12. Switch logic network a 0 b 1

  13. Another switch logic network a r b s

  14. Switch-based mux

  15. Types of switches

  16. Behavior of n-type switch n-type switch has source-drain voltage drop when conducting: • conducts logic 0 perfectly; • introduces threshold drop into logic 1. VDD VDD - Vt VDD

  17. n-type switch driving static logic Switch underdrives static gate, but gate restores logic levels. VDD VDD - Vt VDD

  18. n-type switch driving switch logic Voltage drop causes next stage to be turned on weakly. VDD VDD - Vt VDD

  19. Behavior of complementary switch • Complementary switch • Products full-supply voltages for both logic 0 and logic 1: • n-type transistor conducts logic 0; • p-type transistor conducts logic 1.

  20. Charge sharing • Values are stored at parasitic capacitances on wires:

  21. 0 0 0 Charge sharing example 1 1 1 1

  22. DCSL gate

  23. MTCMOS gate

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