waiting in a sequence

[ljepson74]

*,

 

 Is one of these ways to have a sequence wait on an event preferred?  If so, why?

 

 The following are code snippets from inside a sequence.

 

1) Create transaction and engage w/ driver, then wait for event.

      `uvm_create(req)

      start_item(req);

      m_state.wait_on_smthg();   // <--- wait here

 

2) Wait for event, then proceed

      m_state.wait_on_smthg();  // <--- wait here

      `uvm_create(req)

      start_item(req);

 In this case, the event being waited for is that data of a certain type is available.

 

[David Black]

There are two premises you need to consider:

1. Sequences are supposed to generate transactions in a manner that allows for late randomization. This allows randomization to use current state of the system to guide/constrain the randomization. When you return from start_item(), the driver wants it's transaction request to be honored ASAP.
2. Drivers are typically slaved to hardware. If your sequence blocks (waits), then the driver needs to be written in a manner that doesn't block the way seq_item_port.get_next_item() does. In other words, sequences with waits have implications on the driver.

GIven the above, I would say #2 is preferable because the wait doesn't come after start_item() returns and cause delays in the randomization. In either case, since you have a wait in the sequence, you should be certain to design the driver to be tolerant of those waits. Often this means using seq_item_port.try_next_item().

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[xzcdream]

i wanna to know that how to control the timing between two transactions，and can i send a transaction with non-random data? thanks