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ispGDX Process Flow
Pin Assignment Guidelines
If you want to pre-place signals (not recommended unless pinout configuration is important), follow these
guidelines:
•
Do not place large equations to macrocells or pins at the beginning or end of a block.
•
Signals that share many common inputs should generally be grouped in the same block (the Partitioner
does this automatically). Signals that do not share many common inputs should generally be distributed
across several blocks to avoid overburdening the switch matrix for a single block.
Large Functions at the End of a Block
The macrocells at the end of a block have access to fewer product terms than other macrocells.
•
Cell number 0, the first cell in all devices, can access the product term clusters from adjacent,
higher-numbered cells, but it cannot access any lower-numbered cells (cell 0 being the lowest-numbered
cell in the block).
•
The last cell in a block can access the product term cluster from the adjacent lower-numbered cell, but it
cannot access any higher-numbered cells.
If signals have not been assigned to macrocells, the Fitter will find a macrocell replacement for all the
signals that satisfy their product term requirements.
Adjacent Macrocell Use
In MACH devices, adjacent macrocells can share clusters. Therefore, with designs having equations that use
a high number of product terms, it is a good idea not to place them in adjacent macrocells.
Modifying Assignments
You can use the Location Assignment dialog box of the Constraint Editor to modify the current location
assignment. Modifications can also be made directly in the Pin Attributes sheet of the Constraint Editor.
Deleting Assignments
You can delete project assignments via the Constraint Editor. To do this, select the entire row whose existing
assignment(s) you want to delete. From the Edit menu, select Delete Row(s). You may also right-click and
select Delete Row(s). In cases where you no longer want any of the current assignments, you can delete all
of them at the same time.
Ignoring Assignments
There may be times when you want to ignore, but not delete, the current assignments. For example, after you
complete a design, you may want to try fitting it into a different device. In this case, the current pin
assignments may not be valid for the new device. The ispLEVER software lets you ignore current
constraints for the next Fitter run.
Slew Rate Control
For the majority of Lattice devices, you can set the slew rate to either Slow or Fast. By default, the slew rate
is set to Fast. However, changing it to Slow can result in less board noise.
Optimization
If your design failed to fit, or it did not meet your performance criteria, you can apply optimization
procedures your design again.
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ispLEVER 4.0