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How to create SystemVerilog verification environment?

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Sameh El-Ashry
Sameh El-Ashry

Basic knowledge for the verification engineer to learn the art of creating SystemVerilog verification environment. Starting from the specifications extraction till coverage closure.

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SystemVerilog For Verification How to create a testbench SystemVerilog Environment ? Sameh El-Ashry Senior Digital Verification Engineer
S. ElAshry © 2020 PHASES OF VERIFICATION Analyze Coverage Extract Code Coverage Writing Tests Building Testbench Verification Plan
S. ElAshry © 2020 Building Testbench ▪ In this phase, the verification environment is developed. ▪ Each verification component can be developed one by one or if more than one engineer is working it can be developed parallel. ▪ Writing the coverage module can be done at any time. It is preferred to write down the coverage module first as it gives some idea of the verification progress.
S. ElAshry © 2020 Writing Tests ▪ After the TestBench is built and integrated to DUT, it's time for validating the DUT. ▪ Initially in Constrained Driven Verification, the test are ran randomly till some 70 % of coverage is reached or no improvement in the coverage for 1 day simulation. ▪ By analyzing the coverage reports, new tests are written to cover the holes. In these tests, randomization is directed to cover the holes. ▪ Then finally, the hard to reach scenarios, called as corner cases have to be written in directed verification fashion. Of course, debugging is done in parallel and DUT fixes are done.
S. ElAshry © 2020 Analyze Code Coverage ▪ Once you have achieved certain level of functional coverage, then start for the code coverage. ▪ For doing code coverage, the code coverage tools have option to switch it on. And then do the simulation, the tool will provide the report. ▪ Finally analyze both functional coverage and code coverage reports and take necessary steps to achieve coverage goals. ▪ Run simulation again with a different seed, all the while collecting functional coverage information.
S. ElAshry © 2020 Case Study_1: ALU Block Diagram → Extracting testplan (start point)
S. ElAshry © 2020 Specifications Extraction (Design Requirements)
S. ElAshry © 2020 Specifications Extraction (Verification Requirements)
S. ElAshry © 2020 Verilog Environment For ALU (Modules)
S. ElAshry © 2020 SystemVerilog Environment For ALU (OOP) DUT SystemVerilog Interface Scoreboard stimulus Driver Monitor Coverage Environment Test Base Top Mailbox Mailbox
S. ElAshry © 2020 Case Study_2: ONES COUNTER EXAMPLE ▪ Following example is TestBench for ones counter. ▪ It has some verification components which are required. ▪ The intention of showing this example is to make you familiar with some steps required while building verification environment and to help you to understand the flow of System Verilog environment.
S. ElAshry © 2020 Specification of ones counter (start point) ▪ Ones Counter is a Counter which counts the number of one's coming in serial stream. ▪ The Minimum value of the count is "0" and count starts by incriminating one till "15". After "15" the counter rolls back to "0". ▪ Reset is also provided to reset the counter value to "0". Reset signal is active negedge. ▪ Input is 1 bit port for which the serial stream enters. Out bit is 4 bit port from where the count values can be taken. ▪ Reset and clock pins also provided.
S. ElAshry © 2020 4 bit counter using four D flip-flops Count[0] Count[1] din
S. ElAshry © 2020 The following is the RTL code of ones counter with bugs module dff(clk,reset,din,dout); input clk,reset,din; output dout; logic dout; always@(posedge clk,negedge reset) if(!reset) dout <= 0; else dout <= din; endmodule module ones_counter(clk,reset,data,count); input clk,reset,data; output [0:3] count; dff d1(clk,reset,data,count[0]); dff d2(count[0],reset,~count[1],count[1]); dff d3(count[1],reset,~count[2],count[2]); dff d4(count[2],reset,~count[3],count[3]); endmodule
S. ElAshry © 2020 Verification Test Plan Template Testcase Name Description Section # in standard Type (positive/ne gative) Status (pass/fail/ hang)
S. ElAshry © 2020 Test Plan For Ones Counter Testcase Name Description Section # in standard Type (positive/nega tive) Status (pass/fail/ hang) test1.v Count should increment from "0" to "15". ( Coverage item) test2.v Count should roolover to "0" after "15". (Coverage transition) test3.v Reset should make the output count to "0", when the count values is non "0". ( Assertion coverage)
S. ElAshry © 2020 Verification Environment Hierarchy ▪ TOP |-- Clock generator |-- Dut Instance |-- Interface |-- Assertion block instance |-- Testcase instance |-- Environment |-- Driver | |-- Stimulus | |-- Covergroup |-- Monitor |-- Scoreboard
S. ElAshry © 2020 SystemVerilog Environment For Ones Counter (OOP) DUT SystemVerilog Interface Scoreboard stimulus Driver Monitor Assertion monitor Env Test case Top reset Data Count Count
S. ElAshry © 2020 COVERAGE DRIVEN CONSTRAINT RANDOM VERIFICATION ARCHITECTURE (CDRV) ▪ Input side of DUT : • Generating traffic streams. • Driving traffic into the design (stimuli). ▪ Output side of DUT: • Checking these data streams • Checking protocols and timing ▪ Collecting both the functional coverage and code coverage information. ▪ Writing deterministic tests and random tests to achieve 100% coverage.
S. ElAshry © 2020 Stimulus ▪ When building a verification environment, the verification engineer often starts by modeling the device input stimulus. ▪ In Verilog, the verification engineer is limited in how to model this stimulus because of the lack of high-level data structures. ▪ Typically, the verification engineer will create an array/memory to store the stimulis. ▪ SystemVerilog provides high-level data structures and dynamic data types for modeling stimulus. ▪ Using SystemVerilog randomization, stimulus is generated automatically. ▪ Stimulus is also processed in other verification components. ▪ SystemVerilog high-level data structures helps in storing and processing of stimulus in an efficient way.
S. ElAshry © 2020 Stimulus Generator ▪ The generator component generates stimulus which are sent to DUT by driver. ▪ Stimulus generation is modeled to generate the stimulus based on the specification. ▪ For simple memory stimulus generator generates read, write operations, address and data to be stored in the address if its write operation. ▪ Scenarios like generate alternate read/write operations are specified in scenario generator. ▪ SystemVerilog provided construct to control the random generation distribution and order. ▪ Generally, generator should be able to generate every possible scenario and the user should be able to control the generation from directed and directed random testcases.
S. ElAshry © 2020 Driver ▪ The drivers translate the operations produced by the generator into the actual inputs for the design under verification. ▪ Generators create inputs at a high level of abstraction as transactions like read write operation. ▪ The drivers convert this input into actual design inputs, as defined in the specification of the designs interface. ▪ If the generator generates read operation, then read task is called.
S. ElAshry © 2020 Monitor ▪ Monitor reports the protocol violation and identifies all the transactions. ▪ Monitors are two types, Passive and active. ▪ Passive monitors do not drive any signals. ▪ Active monitors can drive the DUT signals. ▪ Sometimes this is also referred as receiver. Monitor converts the state of the design and its outputs to a transaction abstraction level so it can be stored in a 'score-boards' database to be checked later on. ▪ Monitor converts the pin level activities in to high level.
S. ElAshry © 2020 Assertion Based Monitor ▪ Assertions are used to check time based protocols, also known as temporal checks. ▪ Assertions are a necessary compliment to transaction based testing as they describe the pin level, cycle by cycle, protocols of the design. ▪ Assertions are also used for functional coverage.
S. ElAshry © 2020 Scoreboard ▪ Scoreboard stores the expected DUT output. ▪ The stimulus generator generated the random vectors and is sent to the DUT using drivers. ▪ These stimuli are stored in scoreboard until the output comes out of the DUT. ▪ When a write operation is done on a memory with address 101 and data 202, after some cycles, if a read is done at address 101, what should be the data?. ▪ The scoreboard recorded the address and data when write operation is done. Get the data stored at address of 101 in scoreboard and compare with the output of the DUT in checker module. ▪ Scoreboard also has expected logic if needed.
S. ElAshry © 2020 Coverage ▪ This component has all the coverage related to the functional coverage groups. Environment ▪ Environment contains the instances of all the verification component and Component connectivity is also done. ▪ Steps required for execution of each component is done in this.
S. ElAshry © 2020 Tests ▪ Tests contain the code to control the TestBench features. ▪ Tests can communicate with all the TestBench components. ▪ Once the TestBench is in place, the verification engineer now needs to focus on writing tests to verify that the device behaves according to specification.
S. ElAshry © 2020 Regression Concept ▪ Regression is re-running previously run tests and checking whether previously fixed faults have re-emerged. ▪ New bugs may come out due to new changes in RTL or DUT to unmasking of previously hidden bugs due to new changes. ▪ Each time, when design is changed, regression is done. One more important aspect of regression is testing by generation new vectors. ▪ Usually the seed to generate stimulus is the system time. ▪ Whenever a regression is done, it will take the current system time and generate new vectors than earlier tested. ▪ This way testbench can reach corners of DUT.
S. ElAshry © 2020 How much regression testing?
S. ElAshry © 2020 Coverage Model ▪ Code coverage + Functional coverage Example of Functional Coverage Result
S. ElAshry © 2020 Example of Code Coverage Result
S. ElAshry © 2020 Code Coverage ▪ To check whether theTestbench has satisfactory exercised the design or not? Coverage is used. ▪ It will measure the efficiency of your verification implementation. ▪ Code coverage answers the questions like • Have all the lines of the DUT has been exercised? • Have all the states in the FSM has been entered? • Have all the paths within a block have been exercised? • Have all the branches in Case have been entered? • Have all the conditions in an if statement is simulated?
S. ElAshry © 2020 Functional Coverage ▪ Functional coverage answers questions like • Have all the packets length between 64 to 1518 are used? • Did the DUT got exercised with alternate packets with good and bad crc? • Did the monitor observe that the result comes with 4 clock cycles after read operation? • Did the fifos are filled completely? ▪ Summary of functional coverage advantages: • Functional coverage helps determine how much of your specification was covered. • Functional coverage qualifies the testbenchs. • Considered as stopping criteria for unit level verification. • Gives feedback about the untested features. • Gives the information about the redundant tests which consume valuable cycle. • Guides to reach the goals earlier based on grading.
S. ElAshry © 2020 SystemVerilog Interface Vs BFM ▪ SystemVerilog Interface. ▪ BFM (Bus Functional Model) DUT SV Interface Testbench DUT BFM AXI AHB APB Testbench
S. ElAshry © 2020 Example for a Job Requirements
S. ElAshry © 2020 References ▪ http://www.testbench.in/ ▪ http://www.asic- world.com/systemverilog/index.html ▪ https://verificationguide.com/systemverilog- examples/systemverilog-testbench-example-with- scb/
Thank You ! SystemVerilog Session Presented by Sameh El-Ashry samehelashry@ieee.org https://www.linkedin.com/in/sameh-elashry-22b5603b/

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How to create SystemVerilog verification environment?

  • 1. SystemVerilog For Verification How to create a testbench SystemVerilog Environment ? Sameh El-Ashry Senior Digital Verification Engineer
  • 2. S. ElAshry © 2020 PHASES OF VERIFICATION Analyze Coverage Extract Code Coverage Writing Tests Building Testbench Verification Plan
  • 3. S. ElAshry © 2020 Building Testbench ▪ In this phase, the verification environment is developed. ▪ Each verification component can be developed one by one or if more than one engineer is working it can be developed parallel. ▪ Writing the coverage module can be done at any time. It is preferred to write down the coverage module first as it gives some idea of the verification progress.
  • 4. S. ElAshry © 2020 Writing Tests ▪ After the TestBench is built and integrated to DUT, it's time for validating the DUT. ▪ Initially in Constrained Driven Verification, the test are ran randomly till some 70 % of coverage is reached or no improvement in the coverage for 1 day simulation. ▪ By analyzing the coverage reports, new tests are written to cover the holes. In these tests, randomization is directed to cover the holes. ▪ Then finally, the hard to reach scenarios, called as corner cases have to be written in directed verification fashion. Of course, debugging is done in parallel and DUT fixes are done.
  • 5. S. ElAshry © 2020 Analyze Code Coverage ▪ Once you have achieved certain level of functional coverage, then start for the code coverage. ▪ For doing code coverage, the code coverage tools have option to switch it on. And then do the simulation, the tool will provide the report. ▪ Finally analyze both functional coverage and code coverage reports and take necessary steps to achieve coverage goals. ▪ Run simulation again with a different seed, all the while collecting functional coverage information.
  • 6. S. ElAshry © 2020 Case Study_1: ALU Block Diagram → Extracting testplan (start point)
  • 7. S. ElAshry © 2020 Specifications Extraction (Design Requirements)
  • 8. S. ElAshry © 2020 Specifications Extraction (Verification Requirements)
  • 9. S. ElAshry © 2020 Verilog Environment For ALU (Modules)
  • 10. S. ElAshry © 2020 SystemVerilog Environment For ALU (OOP) DUT SystemVerilog Interface Scoreboard stimulus Driver Monitor Coverage Environment Test Base Top Mailbox Mailbox
  • 11. S. ElAshry © 2020 Case Study_2: ONES COUNTER EXAMPLE ▪ Following example is TestBench for ones counter. ▪ It has some verification components which are required. ▪ The intention of showing this example is to make you familiar with some steps required while building verification environment and to help you to understand the flow of System Verilog environment.
  • 12. S. ElAshry © 2020 Specification of ones counter (start point) ▪ Ones Counter is a Counter which counts the number of one's coming in serial stream. ▪ The Minimum value of the count is "0" and count starts by incriminating one till "15". After "15" the counter rolls back to "0". ▪ Reset is also provided to reset the counter value to "0". Reset signal is active negedge. ▪ Input is 1 bit port for which the serial stream enters. Out bit is 4 bit port from where the count values can be taken. ▪ Reset and clock pins also provided.
  • 13. S. ElAshry © 2020 4 bit counter using four D flip-flops Count[0] Count[1] din
  • 14. S. ElAshry © 2020 The following is the RTL code of ones counter with bugs module dff(clk,reset,din,dout); input clk,reset,din; output dout; logic dout; always@(posedge clk,negedge reset) if(!reset) dout <= 0; else dout <= din; endmodule module ones_counter(clk,reset,data,count); input clk,reset,data; output [0:3] count; dff d1(clk,reset,data,count[0]); dff d2(count[0],reset,~count[1],count[1]); dff d3(count[1],reset,~count[2],count[2]); dff d4(count[2],reset,~count[3],count[3]); endmodule
  • 15. S. ElAshry © 2020 Verification Test Plan Template Testcase Name Description Section # in standard Type (positive/ne gative) Status (pass/fail/ hang)
  • 16. S. ElAshry © 2020 Test Plan For Ones Counter Testcase Name Description Section # in standard Type (positive/nega tive) Status (pass/fail/ hang) test1.v Count should increment from "0" to "15". ( Coverage item) test2.v Count should roolover to "0" after "15". (Coverage transition) test3.v Reset should make the output count to "0", when the count values is non "0". ( Assertion coverage)
  • 17. S. ElAshry © 2020 Verification Environment Hierarchy ▪ TOP |-- Clock generator |-- Dut Instance |-- Interface |-- Assertion block instance |-- Testcase instance |-- Environment |-- Driver | |-- Stimulus | |-- Covergroup |-- Monitor |-- Scoreboard
  • 18. S. ElAshry © 2020 SystemVerilog Environment For Ones Counter (OOP) DUT SystemVerilog Interface Scoreboard stimulus Driver Monitor Assertion monitor Env Test case Top reset Data Count Count
  • 19. S. ElAshry © 2020 COVERAGE DRIVEN CONSTRAINT RANDOM VERIFICATION ARCHITECTURE (CDRV) ▪ Input side of DUT : • Generating traffic streams. • Driving traffic into the design (stimuli). ▪ Output side of DUT: • Checking these data streams • Checking protocols and timing ▪ Collecting both the functional coverage and code coverage information. ▪ Writing deterministic tests and random tests to achieve 100% coverage.
  • 20. S. ElAshry © 2020 Stimulus ▪ When building a verification environment, the verification engineer often starts by modeling the device input stimulus. ▪ In Verilog, the verification engineer is limited in how to model this stimulus because of the lack of high-level data structures. ▪ Typically, the verification engineer will create an array/memory to store the stimulis. ▪ SystemVerilog provides high-level data structures and dynamic data types for modeling stimulus. ▪ Using SystemVerilog randomization, stimulus is generated automatically. ▪ Stimulus is also processed in other verification components. ▪ SystemVerilog high-level data structures helps in storing and processing of stimulus in an efficient way.
  • 21. S. ElAshry © 2020 Stimulus Generator ▪ The generator component generates stimulus which are sent to DUT by driver. ▪ Stimulus generation is modeled to generate the stimulus based on the specification. ▪ For simple memory stimulus generator generates read, write operations, address and data to be stored in the address if its write operation. ▪ Scenarios like generate alternate read/write operations are specified in scenario generator. ▪ SystemVerilog provided construct to control the random generation distribution and order. ▪ Generally, generator should be able to generate every possible scenario and the user should be able to control the generation from directed and directed random testcases.
  • 22. S. ElAshry © 2020 Driver ▪ The drivers translate the operations produced by the generator into the actual inputs for the design under verification. ▪ Generators create inputs at a high level of abstraction as transactions like read write operation. ▪ The drivers convert this input into actual design inputs, as defined in the specification of the designs interface. ▪ If the generator generates read operation, then read task is called.
  • 23. S. ElAshry © 2020 Monitor ▪ Monitor reports the protocol violation and identifies all the transactions. ▪ Monitors are two types, Passive and active. ▪ Passive monitors do not drive any signals. ▪ Active monitors can drive the DUT signals. ▪ Sometimes this is also referred as receiver. Monitor converts the state of the design and its outputs to a transaction abstraction level so it can be stored in a 'score-boards' database to be checked later on. ▪ Monitor converts the pin level activities in to high level.
  • 24. S. ElAshry © 2020 Assertion Based Monitor ▪ Assertions are used to check time based protocols, also known as temporal checks. ▪ Assertions are a necessary compliment to transaction based testing as they describe the pin level, cycle by cycle, protocols of the design. ▪ Assertions are also used for functional coverage.
  • 25. S. ElAshry © 2020 Scoreboard ▪ Scoreboard stores the expected DUT output. ▪ The stimulus generator generated the random vectors and is sent to the DUT using drivers. ▪ These stimuli are stored in scoreboard until the output comes out of the DUT. ▪ When a write operation is done on a memory with address 101 and data 202, after some cycles, if a read is done at address 101, what should be the data?. ▪ The scoreboard recorded the address and data when write operation is done. Get the data stored at address of 101 in scoreboard and compare with the output of the DUT in checker module. ▪ Scoreboard also has expected logic if needed.
  • 26. S. ElAshry © 2020 Coverage ▪ This component has all the coverage related to the functional coverage groups. Environment ▪ Environment contains the instances of all the verification component and Component connectivity is also done. ▪ Steps required for execution of each component is done in this.
  • 27. S. ElAshry © 2020 Tests ▪ Tests contain the code to control the TestBench features. ▪ Tests can communicate with all the TestBench components. ▪ Once the TestBench is in place, the verification engineer now needs to focus on writing tests to verify that the device behaves according to specification.
  • 28. S. ElAshry © 2020 Regression Concept ▪ Regression is re-running previously run tests and checking whether previously fixed faults have re-emerged. ▪ New bugs may come out due to new changes in RTL or DUT to unmasking of previously hidden bugs due to new changes. ▪ Each time, when design is changed, regression is done. One more important aspect of regression is testing by generation new vectors. ▪ Usually the seed to generate stimulus is the system time. ▪ Whenever a regression is done, it will take the current system time and generate new vectors than earlier tested. ▪ This way testbench can reach corners of DUT.
  • 29. S. ElAshry © 2020 How much regression testing?
  • 30. S. ElAshry © 2020 Coverage Model ▪ Code coverage + Functional coverage Example of Functional Coverage Result
  • 31. S. ElAshry © 2020 Example of Code Coverage Result
  • 32. S. ElAshry © 2020 Code Coverage ▪ To check whether theTestbench has satisfactory exercised the design or not? Coverage is used. ▪ It will measure the efficiency of your verification implementation. ▪ Code coverage answers the questions like • Have all the lines of the DUT has been exercised? • Have all the states in the FSM has been entered? • Have all the paths within a block have been exercised? • Have all the branches in Case have been entered? • Have all the conditions in an if statement is simulated?
  • 33. S. ElAshry © 2020 Functional Coverage ▪ Functional coverage answers questions like • Have all the packets length between 64 to 1518 are used? • Did the DUT got exercised with alternate packets with good and bad crc? • Did the monitor observe that the result comes with 4 clock cycles after read operation? • Did the fifos are filled completely? ▪ Summary of functional coverage advantages: • Functional coverage helps determine how much of your specification was covered. • Functional coverage qualifies the testbenchs. • Considered as stopping criteria for unit level verification. • Gives feedback about the untested features. • Gives the information about the redundant tests which consume valuable cycle. • Guides to reach the goals earlier based on grading.
  • 34. S. ElAshry © 2020 SystemVerilog Interface Vs BFM ▪ SystemVerilog Interface. ▪ BFM (Bus Functional Model) DUT SV Interface Testbench DUT BFM AXI AHB APB Testbench
  • 35. S. ElAshry © 2020 Example for a Job Requirements
  • 36. S. ElAshry © 2020 References ▪ http://www.testbench.in/ ▪ http://www.asic- world.com/systemverilog/index.html ▪ https://verificationguide.com/systemverilog- examples/systemverilog-testbench-example-with- scb/
  • 37. Thank You ! SystemVerilog Session Presented by Sameh El-Ashry samehelashry@ieee.org https://www.linkedin.com/in/sameh-elashry-22b5603b/