Download Combinational Gates 4 - Electrical and Computer Engineering

Topics
Pseudo-nMOS gates.
 DCVS gates.
 Domino gates.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Pseudo-nMOS

Uses a p-type as a resistive pullup, n-type
network for pulldowns.
Always on.
Modern VLSI Design 3e: Chapter 3
a
b
out
0
0
1
0
1
0
1
0
0
1
1
0
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Characteristics
Consumes static power.
 Has much smaller pullup network than
static gate.
 Pulldown time is longer because pullup is
fighting.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Output voltages
Logic 1 output is always at VDD.
 Logic 0 output is above Vss.
 VOL = 0.25 (VDD - VSS) is one plausible
choice.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Producing output voltages
For logic 0 output, pullup and pulldown
form a voltage divider.
 Must choose n, p transistor sizes to create
effective resistances of the required ratio.
 Effective resistance of pulldown network
must be computed in worst case—series ntypes means larger transistors.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Transistor ratio calculation

In steady state logic 0 output:
– pullup is in linear region,Vds = Vout - (VDD VSS) ;
– pulldown is in saturation.

Pullup and pulldown have same current
flowing through them.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Transistor ratio, cont’d.

Equate two currents with pull-down
transistor in saturation and pull-up in linear
region:
– Idp = Idd.

Using 0.5 mm parameters, 3.3V power
supply:
– Wp/Lp / Wn/Ln = 3.9.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
DCVS logic




DCVSL = differential cascode voltage logic.
Static logic—consumes no dynamic or static
power.
Uses latch to compute output quickly.
Requires true/complement inputs, produces
true/complement outputs.

The cascode (sometimes verbified to cascoding) is a universal technique for improving
analog circuit performance, applicable to both vacuum tubes and transistors. The word
was first used in an article by F.V. Hunt and R.W. Hickman in 1939, in a discussion for
application in low-voltage stabilizers. They proposed a cascade of two triodes (first one
with common cathode, the second one with common grid) as a replacement of a
pentode.

http://en.wikipedia.org/wiki/Cascode
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
DCVS structure
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
DCVS operation
Exactly one of true/complement pulldown
networks will complete a path to the power
supply.
 Pulldown network will lower output
voltage, turning on other p-type, which also
turns off p-type for node which is going
down.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
DCVS example
Modern VLSI Design 3e: Chapter 3
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Precharged logic
Precharged logic uses stored charge to help
evaluation.
 Precharge node, selectively discharge it.
 Take advantage of higher speed of n-types.
 Requires multiple phases for evaluation.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino logic

Uses precharge clock to compute output in
two phases:
– precharge;
– evaluate.

Is not a complete logic family—cannot
invert.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino gate structure
Domino OR gate
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino phases
Controlled by clock .
 Precharge: p-type pullup precharges the
storage node; inverter ensures that output
goes low.
 Evaluate: storage node may be pulled down,
so output goes up.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino buffer

Output inverter is needed for two reasons:
– make sure that outputs start low, go high so that
domino output can be connected to another
domino gate;
– protects storage node from outside influence.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino operation
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino effect
Gate outputs fall in sequence:
gate 1
Modern VLSI Design 3e: Chapter 3
gate 2
gate 3
Copyright  1998, 2002 Prentice Hall PTR
Monotonicity

Domino gates inputs must be monotonically
increasing: glitch causes storage node to
discharge.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Output buffer

Inverting buffer isolates storage node.
Storage node and inverter have correlated
values.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Using domino logic

Can rewrite logic expression using De
Morgan’s Laws:
– (a + b)’ = a’b’
– (ab)’ = a’ + b’

Add inverters to network inputs/outputs as
required.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Domino and stored charge
Charge can be stored in source/drain
connections between pulldowns.
 Stored charge can be sufficient to affect
precharge node.
 Can be averted by precharging the internal
pulldown network nodes along with the
precharge node.

Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Homework Sets 3-4
Homework Set 3: Problems 2-13,2-19,2,pp. 106-108
Problems 3-1(c), 3.2(d), 3.7, 3.9, 3.12 pp. 179-181
Due, October 16, 2006.
Homework Set 4: Problems 3-15, 3-16, 3.19(b), 3.20(b), p. 182.
Due, October 23, 2006.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR