[myhdl-list] Signal initialization

[David B. <dav...@fr...>]

There is another issue that I would like to bring up. In the following co=
de:

-------------------------------------------------------------

from myhdl import intbv, Signal, Simulation, instances, delay

def test(a, b, c):
    while 1:
        yield a, b
        c.next =3D a.val + b.val
        print "Test function executed"

def a_gen(a):
    while 1:
        yield delay(10)
        a.next =3D 0

def top():
    a =3D Signal(intbv(0))
    b =3D Signal(intbv(1))
    c =3D Signal(intbv())
    a_gen_i =3D a_gen(a)
    test_i =3D test(a, b, c)
    return instances()


top_inst =3D top()
sim =3D Simulation(top_inst)
sim.run(100)

-------------------------------------------------------------

The "test" function is never executed, because "a" and "b" never change t=
heir
value (always 0 and 1 respectively). It means that if another function us=
es the
"c" output value of "test", it will be wrong.
A workaround would be to initialize "a" like this:

a =3D Signal(intbv(None))

So that "a" changes its value in the "a_gen" function. But then I found o=
ut that
in the general case it causes too many problems because some inputs might=
 not be
initialized (still at "None") when the code is executed, resulting in an =
error.
We can solve this by executing the code only if all the input values are
different than "None". Writing this condition after every "yield" stateme=
nt is
not very beautiful (maybe MyHDL could resume the function only if all the=
 input
values are different than "None"?).
Another workaround would be to initialize "a" like that:

a =3D Signal(intbv(0))

and writing "test" with the "yield" statement at the end:

def test(a, b, c):
    while 1:
        c.next =3D a.val + b.val
        print "Test function executed"
        yield a, b

This way, "test" is always executed at least once. I don't know if "alway=
s_comb"
corresponds to this configuration though.

David.