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RTL: Summary of Forms.

From the point of view of this course, Verilog and VHDL are completely equivalent as register transfer languages (RTLs). Both support simulation and synthesis with nearly-identical paradigms. Of course, each has its proponent's.

Synthesisable Verilog constructs fall into these classes:

However, standards for synthesisable RTL greatly restrict the allowable patterns of execution: they do not allow a thread to leave the module where it was defined, they do not allow a variable to be written by more than one thread and they can restrict the amount of event control (i.e. waiting for clock edges) that the thread performs. The remainder of the language contains the so-called 'non-synthesisable' constructs.

All the time values in the RTL are ignored for synthesis and zero-delay components are synthesisable. For them also to be simulatable in a deterministic way the simulator core implements the delta cycle mechanism.

One can argue that anything written in RTL that describes deterministic and finite-state behaviour ought to be synthesisable. However, this is not what the community wanted in the past: they wanted a simple set of rules for generating hardware from RTL so that engineers could retain good control over circuit structures from what they wrote in the RTL.

Today, one might argue that the designer/programmer should not be forced into such low-level expression or into the excessively-parallel thought patterns that follow on. Certainly it is good that programmers are forced to express designs in ways that can be parallelised, but the tool chain perhaps should have much more control over the details of allocation of events to clock cycles and the state encoding.

RTL synthesis tools are not normally expected to re-time a design, or alter the amount of state or state encodings. Newer languages and flows (such as Bluespec and Kiwi) still encourage the user to express a design in parallel terms, yet provide easier to use constructs with the expectation that detailed timing and encoding might be chosen by the tool.

4: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.