Assertion Based Testing

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# Assertion Based Testing - PowerPoint PPT Presentation

Assertion Based Testing. Testing and verification. Does the design function according to the specifications? Example . Traditional approach. Generate a set of test cases (vectors) Apply to the design.. See if the output is correct I.e. check addition, see if 2+2 = 4;

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## Assertion Based Testing

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### Assertion Based Testing

Testing and verification
• Does the design function according to the specifications?
• Example
• Generate a set of test cases (vectors)
• Apply to the design.. See if the output is correct
• I.e. check addition, see if 2+2 = 4;
• VHDL assert/report statement built to support this activity
Coverage
• How many test cases does it take to check ever possible input in a design..
• Percent Coverage = % of all possible cases actually tested
• Number of cases
• Combinational logic 2#of inputs

(combinatorial explosion)

• Sequential logic 2 (#of inputs) x (#of states)

(combinatorial nuclear holocaust)

• Input/signal timing verification

(even harder)

• What coverage is enough?
• Use designers knowledge, generate functional test.. etc…
• This technique is no longer adequate for complex designs
Assertion based testing
• Verify a design by testing a set of behaviors derived from specifications
• Rather that test 2+2=4, test A+B=R.
• Specfic behaviors are called PROPERTIES
• ASSERTIONS are logical statements about properies (I.e. always true, never true)
Examples

PROPERTIES

• P0: (A+B) = R when ALUop = “0001”
• P1: (A-B) = R when ALUop = “0010”
• P2: ALUop != “1000”

ASSERTIONS

• Assert never Property ALUop = “1000”
• Assert implication Property(A+B) = R when ALUop = “0001”
• Assert implication Property(A-B) = R when ALUop = “0010”
Testing Assertions

clock

TEST

CASE

GENERATOR

DESIGN

UNDER

TEST

ASSERTION

CHECKER

Design

outputs

Design

inputs

Generates a new test case on each clock edge

Tests validity of each assertion on each rising clock edge

Assertions in HDL designer
• Open Verification library
• Map the library COE1502_OVL into your project. It is located at I:\1502\COE1502_OVL
• Find the documentation at

http://www.cs.pitt.edu/~don/coe1502/Reference/OVL.html

• Each library component implements one type of assertion.
• You will need to write VHDL to implement the properties to be tested

Component assert_always instantiated from OVL library

Property written in embedded block

User defined message string set in generic “msg”

Building a Testbench
• A testbench is a structured way of building a framework to test your design

HDL designer creates a new block diagram with the component under test and a new component with the same inputs and outputs but with the directionality of each reversed

Inside the tester component

Test case generator

Outputs to

ALU inputs

Inputs from ALU outputs

Assertion checkers

Partitioned by functional block in ALU

Inside the test case generator

Random Number generators

Fixed 64-bit Component

rand64

Set seed value

And width

Variable 1-32 bit component

randv

Testing signed versus unsigned behaviors

--Antecedent Expression

antecedent_expr <= true when ALUop="1011" else false;

--Consequent Expression

consequent_expr <= true when ( ( A < B) AND  (R = X"0000_0000_0000_0001" )) OR ( ( A >= B) AND

(R= X"0000_0000_0000_0000" )) else false;

Testing the Shifters

antecedent_expr1 <= true when ALUop="1110" else false;

consequent_expr1 <= true when (SHR(A,shamt)) = R else false;

antecedent_expr2 <= true when ALUop="1100" else false;

consequent_expr2 <= true when (SHL(A,shamt)) = R else false;

USE ieee.std_logic_unsigned

USE ieee.std_logic_signed