Blanking Bitstreams - 2020.2 English

Vivado Design Suite User Guide: Dynamic Function eXchange (UG909)

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2020.2 English

A blanking bitstream is a specific type of partial bitstream, one that represents a logical black box. In Vivado this is referred to as a greybox, as it is not completely empty. It removes the functionality of an existing RM by replacing it with new functionality, which consists simply of tie-off LUTs on all appropriate module I/O.

To create a greybox RM, you remove the logical and physical representation of a fully placed and routed design configuration and replace it with tie-off LUTs. Starting with a routed configuration (with the static design locked) in active memory, run these steps:

update_design -cell <foo> -black_box 
update_design -cell <foo> -buffer_ports

The design must be placed and routed to implement the LUTs that have been inserted into the design. Outputs of the greybox RM are tied to ground by default, but can be set to Vcc by setting the HD.PARTPIN_TIEOFF on desired ports.

Compression can be used to greatly reduce the size of blanking bitstreams. Note that these bitstreams still contain, not only the tie-off LUTs, but also any static routing that happens to pass through this region of the FPGA. Blanking bitstreams are generated and named in the same way as standard partial bitstreams, as the greybox variation is saved as another configuration checkpoint.

Advisories had been given for prior versions of Vivado software recommending the use of blanking bitstreams for 7 series and Zynq devices to avoid potential glitching conditions. Starting with Vivado 2016.1, these rare glitching scenarios are automatically avoided by embedding specific blanking events in each partial bitstream. Blanking bitstreams, while still available to remove logic from a RP, are no longer required to avoid any potential glitch events. Automatically embedding blanking events results in an increased size of the partial bit files; compression can be used to reduce these effects.