What a 'logic' you have System Verilog !!

Before we understand the “logic” data type for system Verilog, Lets understand verilog data types “reg” and “wire”. Wire is used to connect gates or modules and are physical wire in a circuit and it must be driven by a continues assignment statements. “Reg” in Verilog is a data object that holds its value from one procedural statement to next. When we say "reg (register datatype)" it does not mean the register in the hardware or a physical register in circuit. This is the common mistake or assumption mostly engineers thinks while learning Verilog.

In System Verilog, it improved the classic “reg” data type so that it can also be driven by continues assignments. The name they given for data type is “logic” in System Verilog. It is 4 state (1, 0, X, Z) System Verilog data type.

Let’s take an example to understand the usage of logic data type

Example :

module Asic_With_Ankit (input logic xyz);

   parameter DELAY;
   logic a, b, c;

 initial begin
    a = 0;
    forever #(DELAY/4) a = ~a
  end

  assign c = ~c;

 endmodule

In above example, you can see statement “a = 0” is procedural assignment while statement “assign c = ~c” is a continues assignment. So the important point to understand here is “SV allowed continuous assignments to logic variables, whereas in Verilog, you can’t use continuous assignments to “reg” variables”

“logic” signal can be used anywhere a “net” used but there is one exception to this, you can not drive logic variable from multiple driver. In these type of cases, variable needs to be a net type such as “wire” so that SV can resolve the multiple values.

Logic type can only have a single drive, it can’t allowed multiple driver. This means we can declare all signal as logic to find if is there any multiple driver issue. Because in this case you should be able to see compilation issue if there is any multiple driver by declaring all signal with type “logic”. Of course for the signal you would like to have multiple drivers shall be declared as net type such as “wire” or “tri”.

So, logic data type is identical to “reg” in every way except in SV it improved reg with logic so that it can also be driven by continues assignment and there would not be any confusion on “reg” data type w.r.t to physical register in hardware J

Enjoy,
ASIC With Ankit

System Verilog Class : No numbers for class name please !

Dear Readers,

Today I am going to share you one limitation with system Verilog class definition. We know Class is the basic feature to understand if we have to learn System Verilog. Class is the basic construct, data type in System Verilog. Some of the class feature I highlighted in my couple of previous blog posts can be found from here and here.

Methodological approach to name system Verilog class and major advantages

As a verification engineer with a mindset of methodological test bench architecture we prefer to name all data types, variables, blocks with meaningful names and their objects so that anybody with little understanding of that environment will be easily able to understand why is that class defined or what is the significant of adding this parameter? Let me give you a small example:

For example, a team of 3-5 engineers have developed a complex SoC or Block level verification environment using system Verilog. Environment is such complex that it has many classes defined to perform certain functional verification. Now with methodological approach usually engineers defined their test environment by classifying various configurable parameters and feature control and transaction based features and they will develop environment to accommodate these features in respected class called configuration class and transaction class respectively. They have given a names for cfg and transaction class to “rx_dma_cntrl_cfg” and “rx_dma_cntrl_tr”. Here “_cfg” represents configuration class while “_tr” represents transaction class which is quite common approach in today’s test bench developments.

Why do we need to follow these type of small-small things?

Well, answer to this question is “there are many advantage of following these type of approach while developing your test environment at the same time it does not cost you much”

Advantages:

1. It helps in debugging the complex test environment as we all know “debugging is not free” and it always consumes amount of time and bandwidth in the verification cycles but we can always try to reduce the debugging efforts by following these small steps.
2. Helps engineer to understand environment structure. This will be more helpful when new person joins in the team to work on this type of developed environment. It will surely reduce the reamp-up time for engineer and can easily pick up the tasks to update/modify/enhance such type of test bench environments.
3. Helps in maintenance, enhancement and for re-usability

Limitation (while naming meaningful name to class)

We cannot name class starting with numeric number like 0,1,2 etc.. ! Class name is not allowed to have numbers as first character. Meaning you can’t give a class name called “8_bit_dma_env” or “8bit_dma_env”. Simulation will throw an error if you use this approach, instead it can be defined like “eight_bit_dma_env” or “dma8_bit_env” this is allowed because you are not using numeric number as first character of class name.

Let’s take an example of small exercises to understand :

program ASIC_With_Ankit;

   class 1_byte_display_test;

   int a;
   byte awa = 8'hA5;

   task display ();

      $display ("Byte value =%h", awa);

   endtask

endclass

initial begin
    1_byte_display_test  bdt;

     bdt = new ();
     bdt.display ();

end
endprogram

Solution :

In above example, you can see the class name uses first character as numberic number “1” which is not allowed in system Verilog and when you try compiling this code, you will see an error given below. But if you try the same example with changing a name to “one_byte_display_test” or “byte1_display_test”, you would be able to compile the same exercises without error

#################### ERROR ######################
-- Compiling program ASIC_With_Ankit
** Error: class.sv(3): near "1": syntax error, unexpected "INTEGER NUMBER", expecting "IDENTIFIER" or "TYPE_IDENTIFIER"
################################################

Hope this is useful information, keep reading “ASIC With Ankit” !

Enjoy !
ASIC With Ankit

Semiconductor Acquisistion - Mergers - Takeover and Impacts

Dear Readers,

Mergers and acquisition are common in today’s global market. If you take a history of any successful big companies in the market for more than 10-15 year, you would see the list of companies they acquired. So now two questions comes in mind. "Companies does acquisitions and become successful in the market? Or Successful companies does acquisition to be in market with profitable approach YoY?". There could be a different answers to these question depending upon various factors. I would be pleased to hear answers/views on these questions. Lets share views and experience.

Let’s assume that acquisition is part and process for successful companies and try to understand what impact does it make on different segments.

These days you might be observing industries’ various deals in terms of either merger, acquisition or takeover with all sets of new brand names. It is a way for companies to acquire technology, products and man power to improve profit and stock price while reducing overall expenses. In last few months, I have heard about many small to big acquisition in 2014. Some of them are given below in table and many more could be in the pipeline or in process.

Some of the recent acquisitions-mergers happened in 2014-15:

Avago	LSI - Broadcom
Inphi	Cortina
Infineon	International Rectifier
Seagate	LSI’s Flash business unit
ON Semiconductor	Aptina Imaging
Qualcomm	Wilocity, Black Sand Tech, HP Patents and CSR plc
Synopsys	Coverity

Impacts:

Lets understand what are the segments where mergers/acquisition could make an impact. There are two major segments where we could see the small to major impact because of acquisition and mergers.

1.       Service/consulting/Contract/Freelance

2.       Employees

Service/consulting/Contract/Freelance:

Service segments who mostly relies on the product based companies for business revenue may face a challenge to gain business. During the merger/acquisition time most of the companies holds/stops their decisions for outsourcing their work to service companies. There are strong reason behind holding these decisions as they expect man power, technology and product acquisition as a part of merger/acquisition deal. So management has to think and restructure all sets of resources to make sure effective and profitable utilization model in place. Only after concluding this model they can either release, postpone, cancel their decision on outsource/contract the work. This could eventually effects service business revenue, consulting and contracting opportunities for freelancers. Would be happy to hear from experts on their views on real impact on service business !

Employees:

During the merger acquisition process there are mostly/at-least two group of employees evolved, coming from the organization with different culture. In this transition phase, most of the employees faces challenges, especially on uncertain future, career. They would also be in phase where they think whose job is on critical edge ! This time frame is critical for employees as they need to make sure they are synced with other culture, people, style and way of work along with their individual performance !  Change is always difficult for employees especially they are not engaged in decision that impact their jobs and career. This could lead to a stress for individual and can impact on personal growth.

I had a talk with couple employees who were in same situation and got a smell that employees in this situation may fear losing their jobs or losing opportunities they had. These type of fear can negatively impact on productivity and eventually companies growth engine if continues. One more important point that I realize is “conflict with competitiveness”. When employees are in fear of losing jobs they are more likely to become competitive with others in the same path and eventually can end up having conflict. Competition is good if it is not creating violence !! J These could be major factors where employees gets affected during merger / acquisitions and would be happy to hear views experience from industry experts.

Thanks,

ASIC With Ankit

System Verilog Assertions (SVA) – Types, Usage, Advantages and Important Guidelines!!

ASICs continue to grow in size and complexities and in this case, traditional verification techniques are not sufficient to achieve verification confidence. In the complex designs, debugging simulations is an ever increasing challenge. To address these challenges assertion based verification is found. Design and Verification engineers can place assertions in design or bind to design which will be useful to monitor, report and take action when incorrect behavior is detected. Assertions are playing major role in test bench development which helps achieving maximum confidence on bug free design. Moreover it can be used in simulations as well as in formal verification.It enables engineers to leverage the strength for block level, subsystem level and for chip level verification in order to reduce the overall effort and efficient verification closure. System Verilog Assertions are setting up a viable and effective standard in the design and verification. An assertion adds an advantage in debugging process and makes complex simulation debug easy.

Below figure on block diagram gives brief idea on where we put the assertions in test bench development. As we can see assertion are placed on module boundaries to signals to monitor DUT interface



The introduction of SVA added the ability to perform immediate and concurrent assertions for Design as well as for Verification. Assertions are used to validate design whether it is working correctly or not. Assertions can be useful to make sure ‘How good is the test case?’ Furthermore, it provides a means to measure the quality of the verification process through the creation of coverage using cover property feature of System Verilog assertion.

In System Verilog there are two types of assertions:

Immediate Assertions

Immediate Assertion as name implies, execute immediately, in zero simulation time. It can be used in initial and always procedures, task and functions. This type of assertions performs a true/false. If the test result is true it executes the pass statement and if the test result is false or unknown, it executes false statement. Pass statements are optional; most engineers don’t prefer to specify pass statement as they are more interested on failures.

For example:

assert_xyx : assert (XYZ) $display (“Pass message”);

else $display (“Fail Message”);

Concurrent Assertions

A concurrent assertion uses a clock to trigger the assertions evaluation. Difference between concurrent and immediate assertions is that concurrent assertions evaluate condition over time, whereas immediate assertions test at the point in time when the assertion is called. This assertions can be specified in initial block, always block or standalone like continues assignment

For example:

property XYZ;

@(posedge clk) disable iff (!rst_n)

ABC |=> ## [0:10] (abc == xyz);

endproperty : XYZ

long_lable_p : assert property (XYZ);

else $error (“Error : failure message”);

long_lable_c : cover property (XYZ);

Assertion Advantages

• Dozens of lines of code can be represented in one line using SVA code.
• Assertions can be controlled and can be disabled at any point during the simulations. SVA can be turned off during the reset or until the simulation reaches the particular event or logic.
• Assertion can have severity levels, failures can be non-fatal or fatal errors.
• SVA can be ignored by synthesis; designer does not need to include translate_on and transalate_off throughout the RTL code.
• Assertions can be used in simulation and formal verification.
• An assertion helps in debugging the complex failure. Debugging using end-to-end checkers requires tracing the error from the external interface back to the source. In complex designs, debugging is almost impossible without the help of intermediate checkers. As assertions pinpoint errors at the source, they can be used as intermediate checkers and debugging can be made much easier and faster.

Who/Where should write assertions?

System Verilog is nothing but an extension of Verilog;it has everything to support Verilog with lots of new features for Verification as well as for design.Usually Verification engineers add assertions to a design after the HDL models have been written which means placing the assertions on module boundaries to signals within the model, or modifying the design models to insert assertions within the code.

Design Engineers can write assertion within a design while the HDL models are being coded. What type of assertions and scenario designer should provide within design need to decide before design work begins. Verification engineer define the assertion scenario during the test bench architecture. System Verilog assertions are mostly hooked up at the DUT interface level where it can continuously check protocol and functional activities.

Assertions controllability and why is it important?

Controllability is very important in Assertion design. There are two methods I called to control the assertions: You can also find the Assertion controlabilty blog post from "SVA Control Method" !

D Method (Dynamic Method)

Dynamic method is very popular where engineers required to control the assertion dynamically based on the test scenario they wants to execute. In today’s complex SoCs and their verification, Dynamic (D) method is popular as we regress the erroneous scenario to make sure DUT behaves accordingly. In this type of simulations engineers can dynamically switch on/off assertions.

System Verilog includes system calls for controlling the execution of assertions during simulations. There are three types of D Method.

1. $assertoff:This system function is used to disable all assertions but allows currently active assertions to complete before being disabled.
2 $asserton:This system function is used to turn all assertions back on
3. $assertkill: This system function used to kill and disable all assertions including currently active assertions.

By using $assertoff, the assertions specified as argument of this function will be turned off until a $asserton is executed. This way you can control assertions dynamically. Using these system tasks you can make your assertions dynamic and based on requirement we can make them enable or disable. Furthermore we can even kill using $assertkill.Dynamic control of Assertions can be used to turn off assertions during reset and initialization or during simulation erroneous protocol behavior.

S Method (Static Method):

To control assertion statically we can use System Verilog’s pre-processing capabilities. In this method assertions are generally ignored during the compilation phase w.r.t to `ifdef. This way we can build a different model by defining a different defines. Pre-processing defines can be applied to any place of the code and thus we can control the assertion as per need.

Example:

`ifdef XYZ_ASSERTION_ON

property XYZ (@posedge awa_clk);

a ##2 b;

endproperty

XYZ_p : assert property (XYZ)

else $fatal (“ERROR : Failed message)

`endif
SVA proved to be a powerful assertion language; it provides several ways to control assertion. There are various reasons why assertions controllability is important, some important points are mentioned below:

• To turn off assertions during reset and initialization or during simulation of erroneous protocol by doing this expected fails/checks can be turned off
• Static control of assertions is used to speed up simulations time by turning off checks or to select appropriate assertion model for a given scenarios.
• Dynamic method gives a control on when and where to start and stop/kill the assertions. It gives powerful controllability on simulation control

Coding Guidelines for SVA

• Design code shall not repeated in assertions, it will not be efficient. By making sure not repetition concept, we can decide on use of concurrent or immediate assertions for assertion check. This can be done during the architectural phase of assertions.
• Occurrence of “X” shall be checked explicitly, because “X” is implicitly mapped to “0”, which might hide a real assertion failure.
• Make assertions message as clear as possible. This will help in debugging the assertions.
• Disable the assertions during resets. Concurrent and Immediate assertions in combinational logic will begin executing at the start of the simulation, before reset and during reset design might not be in the known, stable state. Hundreds of false assertions can be reported during this phase and these false assertions could hide a real failure.
• Assertion shall use the same clock as the design; otherwise synchronization will not work properly.
• Assertion needs to consider the reset same as the design reset. This will avoid clock cycle offset which mostly produces illegal failure.
• Name each assertion with meaningful naming convention; this will help in debugging the failure from the failure message and from the wave form display. If we don’t provide long meaning full ‘label’ to the assertion, only non-descriptive generic name will be displayed in the waveform. By looking at the wave engineers will be unable to identify the problem until they consult the original source code. By adding the long descriptive label, we can see the label name on wave itself and can be easily identify the assertion failure details.
• Since the long label names are visible in waveform, it is good idea to use a label naming convention for each label with “ERROR_” followed by description of what the error message is if the assertion fails.
• Use sequence layer as much as possible because sequences can be instantiated within the sequences and properties while properties can’t be instantiated in sequences.
• Identify implementation specific corner cases using cover properties, this will help ensure DUT is exercised properly

Note : Same article is published on Electronicsmaker.com and can be found from here

Happy reading !
ASIC With Ankit

Class - The Classic Feature - Part II

Dear AWA Readers

Here we go with follow up post on ‘Class – The classical feature’ ! In this post I will try to cover different types of classes in brief for better understanding. There are various types of classes that we use in test bench development.  The usage of class is depends on the requirements.  Let’s understand what different types of classes that we use in system verilog.

Different types of classes:

1. Basic Class
2. Abstract Class
3. Parameterised Class
4. Nested Class
5. Typedef Class

Basic Class:

Basic class is covered in my last post, you can refer It from here.

Abstract Class:

Classes without intending to create any objects of class are called Abstract classes.  These type of classes exists simply as a base class from which other classes can be derived. Virtual class is a temple or place holder for derived classes, abstract class is declared with a key word ‘virtual’

virtual class asic_with_ankit ;

virtual task awa1 ();

endtask

virtual task awa2 ();

endtask

endclass

Methods (tasks/functions) can be declared virtual, if methods are declared as virtual and is overwritten in derived class, return types, numbers and types of its arguments must be same as of virtual method. Virtual method defined in abstract class need not have body, In this case body will have to be defined in non virtual (non abstract) derived class.

Parameterised class

You might be knowing the parameterised modules in verilog/system verilog. Classes can also be parameterised in the same way that modules are. This type of class definition provides a template of an actual class, definition of actual class will be created at the time of class instantiation.

Placeholder/template for parameterized class

class  asic_with_ankit  # (parameter int A = 1);

     bit [A-1 : data];

endclass

Actual class will be created when you instantiate the class with parameter value

asic_with_ankit  #(8)    awa ;

Data type also possible to pass as an parameter.

Class asic_with_ankit  #(type A = bit);

     A   register_bit;

     task register_update  (A  reg_update);

endclass

Nested Class:

Definitions appear inside the definitions of another class as it were a member of other class. In system Verilog you are allowed to use class within another class. Class declarations nested inside a class scope are public and can be accessible outside of the class.

Nested class helps increasing encapsulations and lead to more readable and maintainable code.

class A; 
     class B; // Nested class for a node in a linked list. 
          bit [7:0] reg; 
     endclass 
endclass 

Typedef Class

Sometimes a class variable needs to be declared before the class itself has been declared.

For example,

typedef class ASIC_With_Ankit;

     class  AWA;

          ASIC_With_Ankit   AWA1;

     endclass

“typedef class ASIC_With_Ankit;”  : typedef of class ASIC_With_Ankit allows compiler to process the class before it uses inside the class  AWA. This will avoid the compilation error otherwise  compiler will flag an error.

Hope this will be useful information on understanding of different classes in system verilog and their usage.

Happy reading ! 

ASIC With Ankit