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

[Stephen W. <st...@ic...>]

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Martin Whitaker wrote:
> Stephen Williams wrote:
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>>
>> I've been working on some fixes to the tranifX handling in Icarus
>> Verilog. I fixed a couple bugs, but now the tranif1.v test in the
>> ivtest suite no longer works. I've been puzzling over it, and I now
>> think that the test itself is not valid. The meat of the program
>> is here:
>>
>>    wire pin;
>>    pullup   (weak1) (keep1);
>>    pulldown (weak0) (keep0);
>>    tranif1 (pin, keep1, pin);
>>    tranif0 (pin, keep0, pin);

          [...]

> What you are missing is that tranifs are true (bi-directional) switches. Once 
> you turn one of them on, the keeper can drive "pin" through the switch, so 
> will hold "pin" at its last value until something stronger drives "pin" to 
> another value.

I am *not* missing that tranifs are true bi-directional switches.
Believe me, I've gone through a lot of pain to make them work, so
I'm well aware.

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?

You see, the test is relying on a delta delay between an assignment
of Z to the pin and the tranif enable gates responding. If you were
to solve the value of the island of switches based only on the drivers,
you should see this:

  tranif1 (pin=Z, week1, pin=Z);
  tranif0 (pin=Z, week0, pin=Z);

Tell me what that is without invoking delta delays or state machines
or hysteresis.

Now the following I think is well defined.

  wire pin;
  wire keeper;
  assign #(delta) keeper = pin;
  pullup  (week1) peek1;
  pulldown (week0) peek0;
  tranif1 (pin, keep1, keeper);
  tranif0 (pin, keep0, keeper);

That at least 2 of the big3 get the do-what-I-mean answer to the
original example is freaking me out. Modelsim?

- --
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."
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