Packthrus are used BELs that do not have an associated netlist cell. The tool
typically adds them to legalize routing, allow access to otherwise unroutable elements,
or simplify routing. You can include packthru information in the report by using the
-packthru option.
A packthru LUT uses one output and up to one input. The rest of the LUT remains available for use if needed. A register functions as a packthru only when the site is not needed for any other purpose.
In the following example, a packthru LUT drives a logic 0 into the LOOKAHEAD BEL from the LUT's O6 output. The O5 output remains available, and the tool can map a smaller LUT to use this BEL.
Figure 1. BEL Properties
