# Gate Design - PowerPoint PPT Presentation

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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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### 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

• Simple, effective, reliable; hence ubiquitous.

### Static complementary gate structure

Pullup and pulldown networks:

VDD

pullup

Network(P type)

out

inputs

pulldown

Network(N type)

VSS

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'.

+

out

a

+

out

b

a

+

b

a

out

### 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.

out = [ab+c]’:

invert

symbol

circuit

or

and

### 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

a

a

dummy

c

b

b

c

dummy

### 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.

a

0

b

1

a

r

b

s

### 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

### n-type switch driving static logic

Switch underdrives static gate, but gate restores logic levels.

VDD

VDD - Vt

VDD

### n-type switch driving switch logic

Voltage drop causes next stage to be turned on weakly.

VDD

VDD - Vt

VDD

### 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.

### Charge sharing

• Values are stored at parasitic capacitances on wires:

0

0

0

1

1

1

1