7.2.2 MyHDL generators and trigger objects

MyHDL generators are standard Python generators with specialized yield statements. In hardware description languages, the equivalent statements are called sensitivity lists. The general format of yield statements in in MyHDL generators is:

yield clause [, clause ...]

When a generator executes a yield statement, its execution is suspended at that point. At the same time, each clause is a trigger object which defines the condition upon which the generator should be resumed. However, per invocation of a yield statement, the generator resumes exactly once, regardless of the number of clauses. This happens on the first trigger that occurs.

In this section, the trigger objects and their functionality will be described.

Some MyHDL objects that are described elsewhere can directly be used as trigger objects. In particular, a signal can be used as a trigger object. Whenever a signal changes value, the generator resumes. Likewise, the objects referred to by the signal attributes posedge and negedge are trigger objects. The generator resumes on the occurrence of a positive or a negative edge on the signal, respectively. An edge occurs when there is a change from false to true (positive) or vice versa (negative). For the full description of the Signal class and its attributes, see section 7.2.1.

Furthermore, MyHDL generators can be used as clauses in yield statements. Such a generator is forked, and starts operating immediately, while the original generator waits for it to complete. The original generator resumes when the forked generator returns.

In addition, the following functions return trigger objects:

delay( t): Return a trigger object that specifies that the generator should resume after a delay t.

join( arg [, arg ...]): Join a number of trigger objects together and return a joined trigger object. The effect is that the joined trigger object will trigger when all of its arguments have triggered.

Finally, as a special case, the Python None object can be present in a yield statement. It is the do-nothing trigger object. The generator immediately resumes, as if no yield statement were present. This can be useful if the yield statement also has generator clauses: those generators are forked, while the original generator resumes immediately.

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