VLSI Design Assignment 5
Student: Rishikesh Shinde (2023ht01580)
Professor: Dr. Sanjay Vidhyadharan
LAB Professor: Naveen Kanuri
Mentor: Kiran Dahimiwal
Assignment Topic:
1.What are the any 2 issues in dynamic cmos logic implementation explain clearly with the help of circuit diagrams and
simulations and attach .asc file
2.How you can overcome the 2 problems explain clearly with the help of circuit diagrams and simulations and attach
.asc file
1
CONTENTS
1.What are the any 2 issues in dynamic cmos logic implementation explain clearly with the help of circuit
diagrams and simulations and attach .asc file......................................................................................................2
1.1
1.1.1
1.2
Two Major Issues in Dynamic CMOS Logic Implementation ..............................................................2
Introduction to Dynamic CMOS Logic .............................................................................................2
Issue 1: Clock Feedthrough .................................................................................................................3
1.2.1
Conceptual explanation: .................................................................................................................3
1.2.2
Solutions to Clock Feedthrough ......................................................................................................4
1.2.3
Simulation and Verification ............................................................................................................4
1.2.4
Domino Logic: The Comprehensive Solution ..................................................................................5
1.3
Issue 2: Cascading Dynamic Logic Stages ............................................................................................8
1.3.1
Conceptual explanation: .................................................................................................................8
1.3.2
Problematic circuit (naive cascade): ...............................................................................................8
1.3.3
LTSpice simulation to observe cascading failure: ...........................................................................9
1.3.4
How to fix cascading problems (solutions): ....................................................................................9
1.4
COMPARATIVE ANALYSIS ................................................................................................................. 11
1.5
Conclusion ........................................................................................................................................ 11
1.WHAT ARE THE ANY 2 ISSUES IN DYNAMIC CMOS LOGIC IMPLEMENTATION
EXPLAIN CLEARLY WITH THE HELP OF CIRCUIT DIAGRAMS AND SIMULATIONS
AND ATTACH .ASC FILE
1.1 TWO MAJOR ISSUES IN DYNAMIC CMOS LOGIC IMPLEMENTATION
Dynamic CMOS logic is a powerful circuit design technique that offers significant advantages in terms of
speed, area, and power consumption compared to static CMOS. However, its operation based on
temporary charge storage introduces several critical signal integrity issues. This report provides a
comprehensive analysis of two major problems: Clock Feedthrough and Cascading Problem, along
with detailed circuit diagrams, simulations, and practical solutions.
1.1.1 Introduction to Dynamic CMOS Logic
Dynamic CMOS logic operates in two distinct phases controlled by a clock signal. During the precharge
phase (CLK = 0), a PMOS transistor charges the output node to VDD while the pull-down network
(PDN) is isolated from ground. During the evaluation phase (CLK = 1), the output is conditionally
discharged based on the input logic combination. This architecture requires only N+2 transistors for an
N-input logic function, compared to 2N transistors in static CMOS, resulting in smaller area and faster
operation.[1][2][3][4][5]
Clock feedthrough issue in dynamic CMOS logic shown with circuit diagram and voltage simulation
graph.
Clock feedthrough issue in dynamic CMOS logic causes voltage coupling between clock and output due
to gate-to-drain capacitance.
Despite these advantages, dynamic logic faces several challenges including charge leakage, charge
sharing, capacitive coupling, and clock feedthrough. Among these, charge sharing and clock feedthrough
are particularly critical as they can cause functional failures and limit circuit performance.
1.2 ISSUE 1: CLOCK FEEDTHROUGH
1.2.1 Conceptual explanation:
Clock feedthrough results from capacitive coupling between the clock signal and the dynamic output
node through the gate-to-drain overlap capacitance (Cgd) of the precharge transistor. Every MOSFET
exhibits gate-to-drain and gate-to-source overlap capacitances due to physical gate extension over the
source and drain regions.[22][23][7][24]
When the clock signal transitions rapidly (typical rise times of 0.1-1ns), charge couples through Cgd
according to:
This injected charge causes a voltage change at the output:
𝛥𝑉𝑜𝑢𝑡 = −𝑉𝐷𝐷 ×
For typical values
𝐶𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙
𝐶𝐿 + 𝐶𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙
1.2.2 Solutions to Clock Feedthrough
Domino Logic Buffering: The static CMOS inverter in domino logic provides natural isolation from clock
feedthrough. The inverter's low output impedance absorbs voltage glitches and provides a clean, rail-torail output. This robust solution has made domino logic the preferred implementation in highperformance digital circuits.
1.2.3 Simulation and Verification
All provided LTspice circuit files demonstrate these issues with realistic transistor parameters and can
be simulated to observe charge sharing voltage drops and clock feedthrough spikes. The files include
basic demonstrations, circuits with keeper solutions, and complete domino logic implementations.
Users can modify parameters such as transistor sizes, load capacitances, and clock frequencies to
explore design trade-offs.
1.2.4 Domino Logic: The Comprehensive Solution
Domino CMOS logic represents the most widely adopted solution to dynamic logic issues in modern
VLSI design. By cascading a dynamic CMOS stage with a static inverter, domino logic ensures that all
inputs to subsequent stages are at logic '0' during precharge, allowing only 0→1 transitions during
evaluation. This architecture derives its name from the cascading evaluation behavior—when one stage
evaluates and transitions, it causes the next stage to evaluate, similar to falling dominoes.[5][18][19][20][21]
The inverter buffer provides several critical advantages: it isolates the dynamic node from load
capacitance, provides low output impedance for driving subsequent stages, absorbs clock feedthrough
glitches, and enables unlimited cascading with a single clock phase. While domino logic can only
implement non-inverting functions directly (limiting its applicability for certain logic expressions), its
speed and robustness advantages make it the preferred choice for high-performance datapaths,
arithmetic logic units, and wide fan-in gates in modern processors.
1.3 ISSUE 2: CASCADING DYNAMIC LOGIC STAGES
1.3.1 Conceptual explanation:
Cascading dynamic logic stages (directly connecting the dynamic output of one stage to the input of another
dynamic stage) causes problems because dynamic nodes are precharged and may float between clock
phases. If the next stage's precharge/evaluate timing differs, or if the output is floating, the second stage
may precharge incorrectly or evaluate on a wrong value. This creates race conditions, charge redistribution
and incorrect logic levels.
1.3.2 Problematic circuit (naive cascade):
Stage1 (dynamic NAND) OUT -> directly feeds Stage2 (dynamic NOR) input. Both are clocked by same clock
but thresholds and internal node capacitances lead to incorrect evaluation especially if inputs change during
evaluate.
1.3.3
LTSpice simulation to observe cascading failure:
1.3.4 How to fix cascading problems (solutions):
1) Domino logic: Use non-inverting dynamic stage followed by a static inverter before feeding the next
dynamic stage. The static inverter restores signal to a full-swing logic level and prevents floating
dynamic input. Domino style uses only non-inverting dynamic gates connected in series with static
inverters between stages.
LTSpice steps to implement Domino (recommended fix):
Add static inverter between dynamic stages — Place a small static inverter (pMOS and nMOS appropriately
sized) between OUT1 and the input of stage2. The inverter should be static (powered by VDD) — this
restores full logic swing.
Use non-inverting dynamic gates or add an explicit inverting stage — Domino typically uses dynamic NANDs
(non-inverting chain) and static inverter outputs. Avoid cascading dynamic inverters directly.
NOTE: Domino logic prohibits inverting dynamic gates in series without restoration. If you need complex
functions, consider NP domino or complementary dynamic logic styles.
1.4 COMPARATIVE ANALYSIS
Dynamic CMOS logic offers high switching speed and compact area but suffers from reliability
issues such as charge sharing, leakage, and clock-dependent cascading errors.
This section compares three relevant design approaches — conventional dynamic logic, keeperbased dynamic logic, and domino logic — based on performance, robustness, and power efficiency
1.5 CONCLUSION
Dynamic CMOS circuits provide high performance and reduced area advantages compared to
fully static CMOS implementations.
However, they face challenges such as charge sharing, leakage, and signal degradation when
cascaded. Through simulation and comparative analysis:
•
•
•
Keeper-based dynamic logic proved effective in mitigating charge-sharing effects by
maintaining the stored charge on the dynamic node.
Domino logic demonstrated robust cascading capability with restored signal swing and
improved noise margin.
Proper transistor sizing, clock synchronization, and load balancing are essential to ensure
reliable dynamic operation.
In summary, keeper-based and domino dynamic logic circuits offer an optimal compromise
between speed, power efficiency, and robustness. Their application in high-frequency datapath
blocks such as adders and multiplexers makes them invaluable for modern deep-submicron VLSI
design.