Ministry of Higher Education

Nile Higher Institute For ‫وزاره التعليم العالي‬

Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

Academic Year: 2024-2025

Second Semester

Integrated Circuit Design

Report

MOS Clocking and Timing and

Interconnection Issues

DR: Rehab Mahmoud

Eng: Heba Salah

Group Members

Ahmed Mohamed Ismail 200037

Ahmed Samir Soliman 200096
Mohamed Gamal Abd Elgwad 200049
Ahmed Desoky Gaffer 180149
Mohamed Ashraf Sadat Zater 200044
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

1. Introduction

As the scale of integration in digital circuits increases, especially with the advent
of deep submicron and nanometer technologies, timing and interconnect-related
challenges become more critical. In this report, we delve into the intricacies of
MOS (Metal-Oxide-Semiconductor) clocking and timing mechanisms and
highlight the interconnection issues that limit performance, power efficiency, and
scalability of VLSI circuits.
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2. MOS Clocking and Timing

2.1 The Role of Clocking in MOS Digital Systems

Clocking serves as the heartbeat of synchronous digital circuits. In MOS-based

designs, a central clock signal orchestrates data movement and logic transitions
across sequential elements such as flip-flops and registers. A well-designed
clocking scheme ensures reliable data synchronization and accurate timing across
the chip.

2.2 Timing Components in Sequential Circuits

MOS-based sequential circuits rely heavily on correct timing to function as

expected. Key parameters include:

Setup Time (Tsetup): The minimum interval before the clock edge when data must
be stable.

Hold Time (Thold): The interval after the clock edge during which data must
remain stable.

Propagation Delay (tpd): The delay from input to output of a logic gate.
Clock-to-Q Delay: The time between a clock edge and the output change in a flip-
flop.
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

Understanding these parameters is critical for ensuring no violations that could

result in metastability or incorrect data capture.

2.3 Clock Distribution Networks

Clock signals must be delivered uniformly across all parts of the chip. Designing
clock distribution networks (CDNs) involves:

H-tree structures: Balanced tree architectures used to minimize skew.

Grid-based CDNs: High-performance systems with mesh grids to ensure even

clock delivery.

Low-power design: Using clock gating to reduce unnecessary toggling.

2.4 Clock Skew and Jitter

Clock Skew: The difference in arrival times of the clock signal at different flip-
flops. It can be positive or negative and may lead to:

Hold violations (if skew is negative)

Setup violations (if skew is positive)

Clock Jitter: Short-term variation in the period of a clock signal due to noise,
thermal fluctuations, and power supply instability.

Mitigating these requires careful clock tree synthesis (CTS), shielding, and the use
of phase-locked loops (PLLs) and delay-locked loops (DLLs).

2.5 Timing Analysis

Static Timing Analysis (STA): Determines worst-case delays without using input
vectors.
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

Dynamic Timing Analysis (DTA): Uses simulation with real input patterns for
timing validation.

Path-Based Analysis: Focuses on critical paths which determine the maximum

clock frequency.
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3. Interconnection Issues in MOS VLSI Circuits

3.1 Overview of Interconnect Scaling

As transistor sizes shrink, interconnects do not scale proportionally, leading to

increased resistance (R) and capacitance (C). These parasitic elements contribute to
increased delay, power dissipation, and signal degradation.

3.2 Delay in Interconnects

Interconnect delay becomes significant in sub-65nm processes. The Elmore delay

model is commonly used to estimate interconnect delay:

Where:

= wire resistance (Ω)

= total load capacitance (F)

3.3 Crosstalk and Signal Integrity

Crosstalk: Unintended electromagnetic coupling between adjacent interconnects,

which can cause noise-induced logic errors.

Coupling Capacitance: A major contributor to crosstalk, especially in narrow,

densely packed wires.

Mitigation:

Wire spacing
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

Shielding (using grounded wires between signal lines)

Slew rate control

3.4 Power and Energy Concerns

Dynamic power is heavily influenced by interconnects due to frequent charging

and discharging of parasitic capacitances:

Where:

: switching activity factor

: load capacitance

: supply voltage

: clock frequency

Leakage power also increases with reduced dimensions, further stressing the power
budget.

3.5 Electromigration and Reliability

Ahmed Esmail, [01/05/2025 08:58 ‫]م‬

Electromigration: The gradual displacement of metal atoms due to high current
densities, leading to open or short circuits.

Countermeasures:

Increasing wire width

Reducing current density

Using materials with better electromigration resistance (e.g., copper)

3.6 Design Techniques to Minimize Interconnect Impact
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

Buffer Insertion and Repeater Trees: Break long interconnects into segments with
buffers to reduce delay.

Wire Sizing: Optimizing wire width and spacing for performance and reliability.

Layer Assignment: Using upper metal layers (with lower resistance) for long
global interconnects.

3.7 Technology Solutions

Low-K Dielectrics: Reduce parasitic capacitance between wires.

3D ICs and Through-Silicon Vias (TSVs): Shorten interconnect paths by stacking

dies vertically.

Optical Interconnects: A potential future solution for ultra-high-speed data transfer

with minimal delay and crosstalk.

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4. Case Study: Timing and Interconnects in a 7nm Technology Node

A 7nm FinFET-based microprocessor demonstrates the following challenges:

Over 50% of delay is attributed to interconnects.

Clock distribution consumes up to 30% of dynamic power.

Signal integrity analysis must consider hundreds of simultaneous switching outputs

(SSO).

Solutions implemented:

Advanced clock gating for power reduction.

Use of redundant interconnect paths for fault tolerance.

Adaptive timing control using on-chip sensors and voltage regulators.
Ministry of Higher Education
Nile Higher Institute For ‫وزاره التعليم العالي‬
Engineering & Technology ‫معهد النيل العالي للهندسة والتكنولوجيا‬
Communication & Electronics ‫قسم هندسة االتصاالت وااللكترونيات‬
Department

5. Conclusion

MOS clocking and timing, along with interconnection issues, represent some of the
most significant challenges in modern VLSI design. As technologies push toward
smaller nodes and higher frequencies, careful attention to these aspects is essential.
Designers must employ sophisticated timing analysis tools, interconnect modeling,
and layout optimization techniques to ensure robust, low-power, and high-
performance designs.