DCSL & LVDCSL: A High Fan-in, DCSL & LVDCSL: A High Fan-in, High Performance Differential Current High Performance Differential Current

Switch Logic FamiliesSwitch Logic Families

Dinesh Somasekhaar, Kaushik RoyDinesh Somasekhaar, Kaushik Roy

Presented byPresented byHazem AwadHazem Awad

Differential Cascade Voltage Differential Cascade Voltage Switch LogicSwitch Logic

DCVSL is a ratioed logic style that completely eliminates static currents and provides rail-to-rail swings.It provides much higher functionality with greater fan-in than conventional logic. This high functionality allows the number of logic levels and gate output nodes to be minimized.

However, DCVSL circuits exhibit high power consumption due to the pre-charged differential nature of their outputs.

Differential Current Switch LogicDifferential Current Switch Logic

DCSL is a clocked DCVSL logic style that is capable of implementing gates with high fan-in.

DCSL achieves low power by restricting internal voltage swings in the NMOS evaluation tree without sacrificing speed.

Pre-charged high, Pre-charged low and improved pre-charged low topologies are possible. Pre-charged low has superior performance.

Problems with DCSLProblems with DCSL

Large # of NMOS devices in series forces high supply to threshold voltage ratios (VDD>> 3Vth). Not applicable in current CMOS processes.

Steep output spikes are observed.

Gate is not robust due to its sensitivity to differences in output loading.

LVDCSL was conceived to solve these problems.

Low Voltage Differential Current Low Voltage Differential Current Switch LogicSwitch Logic

New topology uses a pre-discharged NMOS tree with a sensing stage to determine outputs.

Avoids the problem of output spikes by having devices P3 & P4 active at start of evaluation. The strong drive at outputs limits the glitch.

At the start of evaluation, both T3 & T4 are off, hence the entire discharge current flows into the NMOS tree => increased gate robustness.

LVDCSL operationLVDCSL operation

The operation of LVDCSL can be divided into three stages:

A) Pre-charge State (CLK# going high and CLK low).

B) Evaluate State (CLK going high and CLK# low)

C) Stable Outputs state (CLK high, CLK# low)

LVDCSL Operation: Pre-Charge StateLVDCSL Operation: Pre-Charge State

CLK # goes high, CLK goes low => T5 & T6 turn ON => T5 turns P4 ON, T6 turns P3 ON.

P3 charges up OUT, P4 charges up OUTBAR.

OUT turns T8 ON, OUTBAR turns T7 ON => a path from VDD to NMOS tree is created.

Since T3 & T4 are OFF, no shunting of current from NMOS tree occurs => ability to work with VDD ~ 2Vth.

LVDCSL Operation: Evaluate StateLVDCSL Operation: Evaluate State

CLK goes high, CLK# goes low =>T1 & T2 turn ON => NMOS tree charges up through T1 & T7, T2 & T8

Assuming NMOS tree has stronger Path on left, node A will be held @ lower voltage than B.

As VB goes > Vth => T3 turns ON => +ve feedback loop drives OUT low and OUTBAR high.

T8 turns OFF => NMOS tree disconnected from OUTBAR.

LVDCSL Operation: Stable Outputs LVDCSL Operation: Stable Outputs StateState

CLK remains high, CLK# remains low.

NMOS tree remains disconnected from OUTBAR => input changes have no effect on OUTBAR.

All inputs to gate may be pre-charged at this state (after evaluation) => allows construction of simple pipe-lining configuration.

Performance comparison Performance comparison w.r.t. Domino logicw.r.t. Domino logic

A 64-bit Carry-Look-Ahead (CLA) adder in 0.35 CMOS was used. VDD = 2.2V, Vth ~ 0.45V.

Critical path in adder is an 8-bit building block composed of two 4-input propagate-generate Domino gates followed by 2 input static CMOS.

Domino Delay (not including static gates) was 210 p.s..

LVDCSL implemented critical path in a single 8- bit stage. Speed higher than one 4 input Domino gate.

Cont. Performance comparison Cont. Performance comparison w.r.t. Domino Logicw.r.t. Domino Logic

LVDCSL requires a setup time (100ps) unlike Domino but still manages a delay of 0.33ns compared to 0.55ns in Domino => 40% improvement

Domino consumed 8mA current during pre-charge state. LVDCSL consumed 4.1mA current during both pre-charge and evaluation states.

Overall current consumption of LVDCSL is 0.873mA compared to 1.131mA in Domino => 22% improvement.

Reasons for improved Reasons for improved performanceperformance

Higher functionality of LVDCSL allows the two 4-input Domino stages to be combined into one single 8-input stage (not always possible though)

Shorter delays are possible in single LVDCSL stages as opposed to multiple Domino stages.

LVDCSL has much lighter loading @ gate inputs because the NMOS transistors in evaluation tree are small. Also helps with power consumption.

LVDCSL also offers lighter loads on CLK lines.

Conclusion: Features and Conclusion: Features and shortcomings of LVDCSLshortcomings of LVDCSL

LVDCSL features:LVDCSL features:

1. High Performance with large stack height in NMOS tree.

2. Robust gate in spite of use of cross coupled inverter loop (unlike DCSL). Load mismatches @ output up to a factor of 5 are tolerated.

3. Power consumption is limited by decreasing the voltage swings at internal nodes of NMOS tree.

Cont. Conclusion: Features Cont. Conclusion: Features and shortcomings of and shortcomings of

LVDCSLLVDCSL4. Latching output stage which automatically locks out gate inputs once evaluation is complete.

LVDCSL Shortcomings:LVDCSL Shortcomings:

1. High complexity of output stage prevents its use in simple gates. Layout of output stage critical because internal nodes A & B have to be balanced. Very short cycle times not possible.

2. Does require a setup time w.r.t. CLK(unlike Domino).

ReferencesReferences

1. Dinesh Somasekhar, Kaushik Roy, “Differential Current Switch logic: A low power DCVS logic family”, IEEE J. Solid State Circuits, June 1996.

2. Dinesh Somasekhar, Kaushik Roy, “LVDCSL: A High Fan-in, High performance Low Voltage Differential Current Switch logic family”, TVLSI, Dec 1997.