AR# 40089

Design Assistant for XST - Help with Register Balancing


Please refer to this answer record for help with Register Balancing.

Note: This Answer Record is a part of the Xilinx Solution Center for XST (Xilinx Answer 38927). The Xilinx Solution Center for XST is available to address all questions related to XST. Whether you are starting a new design or troubleshooting a problem, use the Solution Center for XST to guide you to the right information.


Register Balancing is an XST option/constraint which enables flip-flop retiming algorithm in Synthesis process. The main goal of Register Balancing is to improve design timing performance in the way of moving flip-flops and latches across logic to increase clock frequency.

Register Balancing has two categories:

  • Forward Register Balancing: move a set of flip-flops that are at the inputs of a LUT to a single flip-flop at its output
  • Backward Register Balancing: move a flip-flop which is at the output of a LUT to a set of flip-flops at its inputs

You can apply Register Balancing constraint globally or to a specific entity, module, signal, flip-flop or clock signal. Refer to Xilinx Answer 39749 for help with applying XST constraints.

Some considerations when using Register Balancing:

  • Register Balancing option is disabled by default because it leads to significant design structure changes.
  • The number of flip-flops in the design can be increased or decreased.
  • Register Balancing may happen across cross-clock domain boundaries. Make sure the cross-clock domain timing paths are constrained in the design.
  • The following constraints influence Register Balancing:
    • Keep Hierarchy: Hierarchy preservation limits FF movements inside the block boundaries
    • KEEP: Keep constraint applied to signals does not allow FFs to cross these signals
    • IOB: Register Balancing will be not applied to the FFs having property "IOB=true"

For more information of Register Balancing, please refer to XST User Guide See (Xilinx Answer 38931).



Answer Number 问答标题 问题版本 已解决问题的版本
38927 面向 XST 的 Xilinx 解决方案中心 N/A N/A


Answer Number 问答标题 问题版本 已解决问题的版本
40085 Design Assistant for XST - Performance Considerations N/A N/A
AR# 40089
日期 12/15/2012
状态 Active
Type 综合文章