Abstract

It’s a debatable topic whether to do timing annotated Analog Mixed Signal (AMS) Verification or use Digital methods like Logic design, Timing Libraries and other methods which simplifies simulation results and are less time consuming at the cost of Accuracy.

Use of Spice Models & Behavioral Modeling for Analog Design and RTL for Digital Block can accurately verify the functionality at (AMS) Mixed Signal SoC Verification.

Increasing trend towards Digital, Real Benefits lies in Mixed Signal Design checks and are necessary for interface checks, accurate power consumption calculation, I/O speed characterization and Accurate settling dynamics understanding. Calculations of actual resolution.

First Pass Silicon success as the ONLY goal considering today’s economic conditions.

1. Introduction

Moore's law for number of transistors on Integrated Circuits(IC) will double in every two years, while we are better in terms of number of transistors on IC & technology node of 20nm in most of Designs today, Speed is a concern at which IC operates and still far way and Problem lies in difficulty in Design & Verification of Complex Mixed Signal Circuits which operates greater than ten’s of Giga Hertz. Fast Simulator is required to lessen simulation time while not compromising on accuracy of Results. AMS verification at SoC Level is necessary to confirm interface operations at the Boundary of Analog, Digital and RF Circuits.While the world is going towards Digital, Importance of Analog & Mixed Signal Verification holds good to benefit real world applications.

As the designs are growing in complexity, efforts to design, verify them are also increasing tremendously and require complex flow and environment, high degree of automation to accelerate the design process with high accuracy. Increasing usage of Electronic Design Automation (EDA) tools in the verification flow to support this high degree of automation.

Analog & Digital components are integrated to form a System-on-Chip (SOC). Interfaces (or hook-up) of these modules verified at SOC level Mixed Signal Verification. Analog signals are continuous in time and Amplitude while Digital Signals are Discrete in time & Amplitude. Direct verification methods and using assertion-based verification (ABV) and metric-driven verification (MDV) have been used traditionally to verify Mixed Signal Designs.

Spice Modeling for Analog Designs which helps create equivalent of Analog Circuit Design using simple passive elements resistor, capacitor, Dependant & Independent Sources along with Behavioral Modeling can fasten the speed of Simulation giving the required accuracy.

Important Blocks like Analog to Digital Converter (ADC) and Digital to Analog Converter (DAC) are interface between Real World Analog Signals and Digital Processing World.

2. Analog & Mixed Signal (AMS) Verification “Needs & Ways”

Waves of sound, light, temperature and pressure fluctuations, and shifting magnetic fields are real World signal are Analog. Applications like telecommunication, Consumer Electronics, Computer & related equipments, automotive systems; Bio Medical Instrumentation requires Mixed Signal Circuits and requires Complex Analog & Mixed Signal Design & AMS Verification at SoC.

Setting and bringing up test bench for High Complex Mixed Signal Design is difficult and time consuming might take days to complete simulation run for complex designs like Phase Locked Loop (PLL). Debugging post simulation waveform dumps is bit involved and need prior exposure to give timely results.

Use of Behavioral models & Spice Models in place of Analog Macros & timing data closure libraries for GPIO, USB Interfaces achieve high accuracy & quick simulation.

Simulator speeds have been increasing but still not enough to match with design complexity. Memory optimized signal dump formats will reduce the system memory cost. Coverage metrics can be defined in the tools to measure coverage on timing paths especially for cross clock domain.

• Simulation “truths”:---(Usage of a simulator) x (Common sense) = Constant
• Simulators are only as good as the models and the knowledge of those models by the designer
• Simulators are only good if you already know the answers

3. Conclusions

Digital Circuits are important but can not fully substitute Analog Design as real world signals are Analog eventually locates need for Analog & Mixed Signal (AMS) Design and AMS Verification at SoC Level. Spice Models and Behavioral Modeling for Analog Design helps reduce Simulation time & achieve accurate results and avoid silicon re-spin.  In today’s economic conditions when limited designs are getting worked out. “First pass silicon is the ONLY goal”.

Digital Signal have Analog signal embedded into it while transition from ‘0’ to ‘1’ or ‘1’ to ‘0’ and having purely Digital Signal is impossible. Differentiation lies in Analog Portion or Continuous signal in Digital data is very small compared to steady state and transition from ‘0’ to ‘1’ is definitely Analog & Continuous in time & Amplitude.

While Design have both Analog & Digital Blocks, AMS Verification is necessary to ensure proper Function of SoC & assure First Pass Silicon Success.

AMS Verification is a last opportunity in SOC flow to fix any verification or design issue.  It’s worth taking pains in doing initial setup and overcoming Analog & Mixed Signal overheads. Using AMS Verification is highly relevant today and are very much recommended as sign-off for verification.

Authored by: Santosh Narawade