An Introduction to Digital Systems Simulation Paolo PRINETTO Politecnico di Torino (Italy) University of Illinois at Chicago, IL (USA) Lecture 10.1

Goal  This lecture presents a brief introduction to digital system simulation  The use of a simulation tool for Validation & Verification is targeted as well.

Prerequisites  Module 4

Homework  No particular homework is foreseen

Further readings  Students interested in a deeper knowledge of simulation algorithms can refer, for instance, to:  M. Abramovici, M.A. Breuer, A.D. Friedman: “Digital System Testing and Testable Design (revised printing),” IEEE Press, Piscataway, NJ (USA), 1990 (chapter 3, pp )

Outline  Introduction  Simulation tool architecture  Using a simulation tool for Validation & Verification.

Simulating a system consists in analyzing the behavior of a model of the system itself. Simulation

Simulating a system consists in analyzing the behavior of a model of the system itself. Simulation Usually extracted from the system description

Simulating a system consists in analyzing the behavior of a model of the system itself. Simulation Usually generated from the system description According to its accuracy, different information can be extracted from the simulation experiment

Status Simulation is the industrial usual approach for:  project debugging  verification  validation  design rule checking  performances analysis  timing analysis  …

Outline  Introduction  Simulation tool architecture  Using a simulation tool for Validation & Verification

Circuit description InputWaveformsGeneratorSimulationCommands Output Waveforms Analyzer Simulation Engine Libraries

Circuit description InputWaveformsGeneratorSimulationCommands Output Waveforms Analyzer Simulation Engine Libraries Computes the output waveforms of the system, starting from the circuit description and the input waveforms

Circuit description InputWaveformsGeneratorSimulationCommands Output Waveforms Analyzer Simulation Engine Libraries pattern i pattern i < file1.pat 10=1 (200=0 200=1)*2 wfm (1000 = inc by 1\D)*4

Circuit description SimulationCommands Output Waveforms Analyzer Simulation Engine Libraries Waveforminterpreter Inputwaveforms

Circuit description SimulationCommands Output Waveforms Analyzer Simulation Engine Libraries Waveforminterpreter Inputwaveforms |Simulate the circuit RUN sim 10ns cycle 10 step next

Circuit description Output Waveforms Analyzer Simulation Engine Libraries Waveforminterpreter Inputwaveforms Commandinterpreter Simulationcommands

Output Waveforms Analyzer Simulation Engine Libraries Waveforminterpreter Inputwaveforms Commandinterpreter Simulationcommands Design Data Base Networkdescription Networklinker Hardwaremodeller

Output Waveforms Analyzer Simulation Engine Libraries Waveforminterpreter Inputwaveforms Command interpreter Simulation commands Design Data Base Network description Network linker Hardware modeller ABCDE

Simulation Engine Libraries Waveforminterpreter Inputwaveforms Commandinterpreter Simulationcommands Outputwaveforms Waveformanalyzer Design Data Base Networkdescription Networklinker Hardwaremodeller

Simulation Engine Libraries Waveforminterpreter Inputwaveforms Commandinterpreter Simulationcommands Outputwaveforms Waveformanalyzer Design Data Base Networkdescription Networklinker Hardwaremodeller Libraries

Outline  Introduction  Simulation tool architecture  Using a simulation tool for Validation & Verification

Be careful: Exhaustive Simulation is mostly unfeasible

Just an example... Exhaustively simulating a 32 bit adder requires applying 2 65 input values Assuming simulating a single value require 1 ns, the whole simulation process would requires 1,100+ years !!!!

A further example... CPU Intel 8080:  about 120 latches  256 machine instructions  8-bit and 16-bit operands: 3.15  instructions

A further example... CPU Intel 8080:  about 120 latches  256 machine instructions  8-bit and 16-bit operands: 3.15  instructions years 2 · The life of our Universe is estimated 2 · years !

A basic approach to V&V  Identify the set of peculiar aspects, or properties, of the design that need to be checked

A basic approach to V&V  Identify the set of peculiar aspects, or properties, of the design that need to be checked  Organize your V&V operations as a set of independent sessions, each aiming at checking the correctness of some of the above selected peculiar aspects or properties.

A simulation session  For each target aspect or property:  State how to check it  Select the minimum set of input values needed  For each selected value, state the expected output  Identify the most efficient way for:  applying the input values  analyzing the circuit behavior.

A simulation session (cont’d)  Hints:  Simulate the basic behaviors of the system, first  Then, focus on potential critical situations:  Identify border line conditions between alternative behaviors (e.g., change the status of the circuit)  Force the circuit to reach border line conditions

Identify the most efficient way to perform simulation experiments A simulation experiment can be performed in either way:  resorting to a VHDL Test Bench (highly recommended)  controlling the experiment manually, in a highly interactive manner (for expert designers or for fine tuning and debugging).

The basic principle VHDL Description of the target system

The basic principle VHDL Description of the target system Test Bench

The basic principle VHDL Description of the target system Test Bench

The basic principle VHDL Description of the target system Test Bench Implemented as 2 structural Descriptions: - the VHDL Circuit - the Test Bench

The basic principle VHDL Description of the target system Test Bench The entity of the target circuit must be the actual one

Test Bench characteristics  The same Test Bench should be used though the overall design process, from specs to netlist

Test Bench characteristics (cont’d)  The Test Bench is described for simulation purposes, only, and therefore it does not have to be synthesizable  It may thus contains not-synthesizable statements, such as:  timing specification (e.g., wait) to generate the input waveforms  assertions to check the output waveforms and to generate error messages.

Test Bench characteristics (cont’d)  The Test Bench can have different complexities, to perform different tasks during the various V&V steps:  just apply input values:  generated internally  read from external files  apply input values & check output values  save output values on external files ...

Some examples We shall consider some cases.

Case #1  The input values are explicitly specified within the Test Bench process  The correctness of the output values is evaluated analyzing the displayed output waveforms, using the facilities of the simulation tools.

Case #2  The input values and the corresponding expected outputs are stored inside the Test Bench, resorting to an array  The Test Bench process reads from the array the input values, and applies them to the target system  The correctness of the output values is evaluated inside the Test Bench process. Using an assert statement, the output values of the target circuit are compared with the expected ones, stored into the array.

Case #3  The input values and the corresponding expected outputs are stored into two distinct files  First, the Test Bench process reads, from an input file, the input values and applies them to the target system  Then, the Test Bench process writes, on an output file, the output values generated by the target circuit

Case #3 (cont’d)  The correctness of the output values is evaluated externally to the Test Bench process: the generated output file is compared with a file of expected output values, defined by the designer.

Case #3: typical usage  The proposed approach is typically used for both design validation and verification.

Simulation for Design Validation Comparator Input values Simulator Simulation results Expected results