One of the main guidelines of this book is to create a single verifi cation environment that you can use for all tests with no changes. The key requirement is that this testbench must provide a “hook” where the test program can inject new code without modifying the original classes.
Your driver may want to do the following:
- Inject errors
- Drop the transaction
- Delay the transaction
- Put the transaction in the scoreboard
- Gather functional coverage data
Rather than try to anticipate every possible error, delay, or disturbance in the flow of transactions, the driver just needs to “call back” a method that is defined in the top-level test.
The beauty of this technique is that the callback method can be defined differently in every test. As a result, the test can add new functionality to the driver using callbacks, without editing the Driver class.
For some drastic behaviors such as dropping a transaction, you need to code this in the class ahead of time, but this is a known pattern. The reason why the transaction is dropped is left to the callback.
As shown in "Developer code" the Driver::run task loops forever with a call to a transmit task. Before sending the transaction, run calls the pre-transmit callback, if any. After sending the transaction, it calls the post-callback task, if any. By default, there are no callbacks, so run just calls transmit.
You could make Driver::run a virtual method and then override its behavior in an extended class, perhaps MyDriver::run. The drawback to this is that you might have to duplicate all the original method’s code in the new method if you are injecting new behavior. Now if you made a change in the base class, you would have to remember to propagate it to all the extended classes. Additionally, you can inject a callback without modifying the code that constructed the original object.
A callback task is created in the top-level test and called from the driver, the lowest level of the environment. However, the driver does not have to have any knowledge of the test – it just has to use a generic class that the test can extend.
The beauty of this technique is that the callback method can be defined differently in every test. As a result, the test can add new functionality to the driver using callbacks, without editing the Driver class.
For some drastic behaviors such as dropping a transaction, you need to code this in the class ahead of time, but this is a known pattern. The reason why the transaction is dropped is left to the callback.
As shown in "Developer code" the Driver::run task loops forever with a call to a transmit task. Before sending the transaction, run calls the pre-transmit callback, if any. After sending the transaction, it calls the post-callback task, if any. By default, there are no callbacks, so run just calls transmit.
You could make Driver::run a virtual method and then override its behavior in an extended class, perhaps MyDriver::run. The drawback to this is that you might have to duplicate all the original method’s code in the new method if you are injecting new behavior. Now if you made a change in the base class, you would have to remember to propagate it to all the extended classes. Additionally, you can inject a callback without modifying the code that constructed the original object.
A callback task is created in the top-level test and called from the driver, the lowest level of the environment. However, the driver does not have to have any knowledge of the test – it just has to use a generic class that the test can extend.
Limitations of SystemVerilog callback (w.r.t. uvm_callback).
- You cannot control particular nth number of transaction. Callback affects all the transaction or in random manner if you use randomization in extended callback as I had used in above both examples. For example you want initial some (n) transcation to drive without any modification, then for particular (n+1) transaction you want to modified its content. Then again for rest of all transaction you don't want any modification. This is not possible with SystemVerilog callback.
- You cannot Add or Delete callback runtime.
Good Article, thank you.
ReplyDeleteThanks very much for this article, I found it both informative and interesting.
ReplyDeletesprint error 104
As per limitation listed here, can't we use static variable to give id to all transaction and once that nth number of transaction comes, we can do operation as needed? Please give resolution on this.
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