Re: [Iverilog-devel] The tranif1 test is not right?

[Cary R. <cy...@ya...>]

--- On Tue, 10/13/09, Stephen Williams <st...@ic...> wrote:

> In fact, it's their "switch" nature that makes me think
> this test is wrong. Ask yourself:
> 
>  - what happens when "pin" is driven 0?
>  - what happens when "pin" is driven 1?
>  - what happens when "pin" is driven X?
>  - what happens when "pin" is driven Z?
> and
>  - Does it make sense for the answer to any of the above to
>    differ depending on what happened before it?

0 and 1 are real signals, X is an indeterminate real signal
(it's 1 or 0, we just don't know which one), but Z is special,
it is supposed to represent a lack of drive. I don't think
of this as driving Z, but more as not driving anything. In
that context it seems to make sense that the tranif gates
work as they do. If you are no longer driving a signal the
pulls keep it in the previous state.

Cary