What are Program Blocks?
The basic syntaxes of Verilog language provide constructs that
can be used for both logic design and test benching.
From a language perspective, the primary focus of a design is,
of course, how easily the constructs can be synthesized. To make
the design work easier, we often build it up as a hierarchy of
modules and connect ports of two such modules using nets. Once the
hierarchy is correctly built up, the modules can interact with
one another.
While you can design a testbench in a similar way (and, indeed,
it has been done that waysince from the inception of Verilog), a
few minutes of thinking will convince you that the only things
that a testbench should care for are:
- sending input stimulii to the design under test (DUT),
- receiving and verifying the response for the DUT.
Rest of the ways how a design is implemented - hierarchies,
design elements such as registers or pull up wires and so on -
have no real significance for a testbench. Following the
norms of a design while writing a test bench may significantly
increase the turn-around time.
One other problem that often plagues a testbench environment is a
race condition between the design and the testbench. Consider the
folllwing example. Suppose a testbench is required to wait for a
specific response from its DUT. Once it receives the response,
at the same simulation time it needs to send a set of
stimulii back to the DUT. Since Verilog can execute events in
various procedural blocks out-of-order, there is a chance that
the testbench may send out the stimulii even before the response
from the DUT arrives.
Program block is a recognition of these differences in goals of
writing a design and a test bench. It is meant to facilitate
writing of a test bench.
A program block is defined within the program and endprogram
keyword pair. It serves three purposes.
- It provides an entry point to the execution of test
benches. This is somewhat similar to what a module does for
design related constructs.
- It acts as a scope for data defined within this
program block. Once again similar to a module, a program block
can act as a scope for encapsulating program wide data.
- Perhaps the most important aspect of a program block is
that it is a syntactic context that schedules events
in the 'Reactive region'. We will see in few minutes what this
means and how this can be used to avoid race condition.
Next: Creating Programs
