Answers Database

Foundation Simulator: How can I use a Formula to assign Z to a bus?


Record #2620

Product Family:  Software

Product Line:  Aldec

Problem Title:
Foundation Simulator: How can I use a Formula to assign Z to a bus?


Problem Description:
Keywords: stimulus simulate high impedance tristate
	  bidirectional
Urgency:  Standard

General Description:
When defining a formula for a bus, if you use the value [Z],
the bus is driven to a random state (e.g. 23) instead of
high impedance.


Solution 1:

The Foundation Logic Simulator cannot correctly interpret
the bus value [Z] in a formula.

One workaround is to break up the formula into sections that
are valid data and sections that are high impedance, then
reassemble the pieces.

NOTE: Only use this workaround if you want the Z value to
propagate through your simulation.  If you want to release
the assigned stimulator to allow a tristate buffer to drive
the signal, refer to the other resolution.

Example
-------
You want to put the following pattern onto an 8-bit bus
called DATA:  [Z]50 [30]50 [FF]50 [Z]100 [A5]50 [Z]50 [00]50

1. Add the DATA bus to the Waveform Viewer.

2. Select Waveform->Formula->Edit to open the Formula Editor
   dialog.

3. Define the following formulas, clicking on Add after
   entering each formula:

      Z50    <---IMPORTANT: Notice there are no brackets
      [30]50[FF]50
      Z100
      [A5]50
      [00]50

4. In the Waveform Viewer, locate the line for the DATA
   bus.  Click on the DATA line, underneath time 0ns.  The
   DATA signal name should turn green.

5. Select Waveform->Formula->Insert to open the Formula
   Selection dialog.

6. Select the formula you need (Z50 in this case) and click
   OK.

7. Now click on the DATA line, right where the first formula
   ends.

8. Select Waveform->Formula->Insert and insert the next
   piece of the waveform ([30]50[FF]50 in this case).

9. Repeat steps 7 and 8 to construct the entire formula.

If you insert the wrong formula, just select Waveform->
Formula->Replace and select the correct formula.  The
incorrect formula will automatically be deleted.



Solution 2:

If you are trying to put stimulus onto a bidirectional
bus, use a Chip Controlled stimulator instead of putting a
'Z' state into your formula.

A stimulator that is in Chip Controlled mode has a weak
driving strength.  When applied to the output of a tristate
buffer, the stimulator will be active when the buffer is
disabled, but will be overridden whenever the buffer becomes
enabled.

Example
-------
If you originally wanted to define the following formula:

[Z]50 [30]50 [FF]50 [Z]100 [A5]50 [Z]50 [00]50

Define it like this instead:

[30]100 [FF]50 [A5]150 [00]100

Note that each occurence of [Z] in the original formula has
been absorbed into the next bus value.

After assigning this formula to a bus in the Waveform Viewer,
select the bus and then select Signal->Stimulator Mode->
Chip Controlled.



End of Record #2620