RE: [sv-ec] More clocking block issues

Dave,

I agree that this isn't covered, and should be.

> If I have a clockvar output whose target is a variable
> bit vector, and within a given clock cycle I only drive
> a part or bit select of that clockvar, do the undriven
> bits use their previous value?

Similarly for any packed thing.

My personal opinion is that it should be illegal to drive
such selects.  I imagine the clockvar to be "hiding" the
specific structure of the target variable, and I'm entirely
OK with that; clocking blocks already hide many aspects
of the target - its timing, the specific clock expression
with which it's synchronized, whether it's a net or a 
variable - from the testbench.

Suppose, for example, we had a target that was a 
concatenation:

  clocking cb...
    output #1 controls = {Rd, Wr, Mode[4:2]};
  endclocking

What does it mean to do
  cb.controls[2] <= 1'b1;  ??

I'd be much happier not to get enmired in that discussion;
it's already caused much headache on sv-ac where they're 
trying to come up with a way of bit/part-selecting arbitrary
expressions.

For most users, my preferred restriction that a clockvar 
cannot be part-selected or otherwise decomposed is at 
worst a minor inconvenience.  It encourages the idea that
you should prepare your desired clocking drive value in
an auxiliary variable and then drive it out to the 
clockvar all of a piece, which I suspect is pretty good
guidance anyway.  If you want to control things 
independently, put 'em on distinct clockvars.

> A similar question for unpacked structs and arrays: 
> If a member of an unpacked type is selected in a clockvar 
> drive, then should the entire target variable be assigned 
> with previous values of all the other clockvar members? 

Again, I'd prefer it if this were forbidden.  If people 
want individual access to elements of an unpacked thing,
they can create a distinct clockvar for each element.
--
Jonathan Bromley, Consultant

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