transmission gate based circuits. elmore delay (ho) – application of elmore delay to mux design...

Transmission Gate Based Circuits

• Elmore Delay (HO)– Application of Elmore Delay to Mux Design (Ex.

7.4)– Logical Effort of CMOS Transmission Gate (

• Dynamic D-Latch• Dynamic Logic

Distributed RC line as a lumped RC Ladder

Lumped

NMOS TG as a D-Latch

CLK=1, Q=DCLK=1 →0, Qlast is stored on C2

CLK=0, high impedance state.

Problems with NMOS TG

1. Q can only rise to VDD-VT

2. Clock feedthrough at Q when CLK goes low3. The output stored in a high-Z stage after CLK goes low is susceptible to all of the charge lossmechanisms.4. is not available

CMOS TG as a latch

1. Q can only rise to VDD-VT

2. Clock feed through at Q when CLK goes low3. The output stored in a high-Z stage after CLK goes low is susceptible to all of the charge lossmechanisms.4. is not available

CMOS TG with a

1. Q can only rise to VDD-VT

2. Clock feed through at Q when CLK goes low3. The output stored in a high-Z stage after CLK goes low is susceptible to all of the charge lossmechanisms.4. is not available

Use feedback to statically hold the logic value when the latch is off (1)

We can NOT drive a load from internal Q

Use feedback to statically hold the logic value when the latch is off (2)

No Feedback when the latch is ON

Problem & Solution

CLK 𝐶𝐿𝐾

Problem: If D and Qprev are different:Driver + TG1 will drive Q to a different value while INV2 and NMOS of TG2 will drive Q to Qprev

Solution: Size the forward path so that it is stronger than the feedback path.

Adjust VS

• Knob:– χ as defined in EQ. 4.15– Increase (WNLP)/(LNWP)→ Decreased VS.

– Decrease (WNLP)/( LNWP) → Increased VS.

Increase WP to adjust VS

WN/LN=200nm/200nm WP/LP=200nm/200nm

WN/LN=200nm/200nm WP/LP=460nm/200nm

Typical D-Latch Implementation in CMOS

Typical D-Latch Implementation in CMOS

CLK=1

10

Typical D-Latch Implementation in CMOS

CLK=0

Qprev=1

10

Typical D-Latch Implementation in CMOS

CLK 𝐶𝐿𝐾

Qnow=0Qprev=1

1 0

Optional

Typical D-Latch Implementation in CMOS

CLK 𝐶𝐿𝐾

Qnow=1Qprev=0

0 1

Node X may have difficulty transitioning to 1 until is 0.

Optional

Schematic of a TG Based D latch

Simulation of D-Latch

Zoom in to a transition

Positive Edge D Flip-flop

D is only transmitted to the output on the rising edge of CLK

Positive Edge D FF (CLK=0)

Positive Edge D FF (CLK=1)

Dynamic NAND

CLK=0 (Pre-Charged Phase)NMOS is OFF. OUT is charged to VDD.

CLK=1 (Logic Evaluation Phase)NMOS is ON. If either A or B is GND, OUT=VDD. If A=B=1, OUT=GND

Precharge Phase is only a small portion of the clock cycle.

Disadvantage:All dynamic logic circuits require a clock.

General Structure of a Dynamic Gate

Disadvantage:All dynamic logic circuits require a clock

Examples

• Example 7.6• P7.5 (a)• P7.5 (b)

Problem of Domino Logic Gates

1. During the precharge phase, the output voltage is high.2. There is an active path to ground as soon as the foot transistor is turned on. 3. Once an output node has been discharged, it cannot go high until the next prechargephase.

Solution

1. Define each stage as a dynamic gate plus an inverter.

2. The output of each stage is now 0 during precharge. Therefore all NMOS transistors are off during precharge and can only be turned on during the evaluation phase.

Disadvantage: An inverter can not be created!

Domino Cascaded Gates

During the pre-charge phase (Φ=0), Y1, Y2 and Y3 are charged to VDD simultaneously. Φ =0 does not have to last very long since all stages are pre-charged simultaneously.Φ has a high duty cycle.Note: There is no direct current from VDD to GND.

Exercise

c

b

a

clk

clk

VDD

OutX

Solution:

Out A BC

1. NMOS network implements while X implements OUT.2. The output of Inverter implements

Implement the expression

Out AB BC C

Solution

a

b

c

b

clk

clk

VDD

Out

c

Propagation Delay of Domino Cascaded Gates

The propagation delay is determined by:1. The falling edge of the dynamic block2. The rising edge of the inverter

Y1,Y2 and Y3 fall like dominos.

Improve the fall time of a Dynamic Block

• Design a domino stage with a stronger pull-down.– Increase the sizes of NMOS devices.

c

b

a

clk

clk

VDD

Out

The NMOS devices do not have to fight with the pull up network. So the switch voltage is lower. (VTN of the NMOS)

Static Inverter

Domino Gate

Improve the Rise Time of an Inverter

• Design a static inverter with strong pull-up– Increase the size of the PMOS device.• Decrease WNLP/LNWP → Increased Vs of the inverter

Static Inverter

Domino Gate

Logical Effort Comparison

5/3 2/3 assuming that CLK is does notarrive prior to either A or B

Dynamic NOR Gate

• The Dynamic NOR gate is a faster circuit because only one NMOS device is driven

• The pull-down transistors do not fight with the pull-up devices.

Limitations of Domino Logic

• Charge Sharing

Vx(initially)=0V*=(Cout)/(Cx+Cout)VDD

Minimizing the effect of Charge Sharing Using Keepers

The keepers keep VX at VDD and reduce charge sharing to minimum. The keepertransistor is weak enough (small W/L ratioby using a large L) that when X=VDD→GND.NMOS can prevail over weak PMOS.

Disadvantage: large driver requirement of INV

Keepers

X

Enhancement

The INV sees a minimum length device.The effective pull-up strength is controlledby the long device.