1.3.1. SCRUB Mode
The static regions controlled by the CRAM bits from the same programming frame as the PR region continue to operate. All the CRAM bits corresponding to a PR region are overwritten with new data, regardless of what was previously contained in the region.
The SCRUB mode of partial reconfiguration involves re-writing all the bits in an entire LAB column of the CRAM, including bits controlling any part of the static region above and below the PR region boundary being reconfigured. You can choose to scrub the values of the CRAM bits to 0, and then rewrite them by turning on the Use clear/set method along with Enable SCRUB mode. The Use clear/set method is the more reliable option, but can increase the size of your bitstream. You can also choose to simply Enable SCRUB mode.
If there are more than one PR regions along a LAB column, and you are trying to reconfigure one of the PR regions, it is not not possible to correctly determine the bits associated with the PR region that is not changing. For this reason, you can not use the SCRUB mode when you have two PR regions that have a vertically overlapping column in the device. This restriction does not apply to the static bits because they never change and you can rewrite them with the same value of the configuration bit.
If you turn on Enable SCRUB mode and do not turn on Use clear/set method, then the scrub is done in a single pass, writing new values of the CRAM without clearing all the bits first. The advantage of using the SCRUB mode is that the programming file size is much smaller than the AND/OR mode.
This is the floorplan of a FPGA using SCRUB mode, with two PR regions, whose columns do not overlap.
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