Throughout the design cycle, you must verify that the logic level distribution fits the clock frequency goals for the target Xilinx device family and device speed grade. Although a limited number of paths with a high number of logic levels do not always introduce a timing closure challenge, you can improve the timing QoR by optimizing the longest paths in the design with the Vivado synthesis retiming option.
Using the retiming option globally is usually runtime intensive and can negatively impact power. Therefore, Xilinx recommends that you identify a specific hierarchy with violations on paths with a high number of logic levels after synthesis or with optimal placement. When the paths in the fanin or fanout of the longest paths have fewer logic levels and are contained within a small or medium hierarchical module, you can use the BLOCK_SYNTH.RETIMING block-level synthesis strategy.
The following figure shows a critical paths with 5 LUTs, constrained by a 600 MHz clock. The REG2 destination flop drives a timing path with a single LUT that is included one hierarchy up from REG2.
In addition to using the Schematic window
in the Vivado IDE, you can use the
report_design_analysis
-logic_level_distribution command to review the
distribution of logic levels for specific paths. This allows you to
determine how many paths need to be rebalanced to improve the timing
QoR.
You can use the retiming_forward and retiming_backward attributes available in Vivado synthesis to control the optimization on a
specific register or a path. Using these attributes applies retiming optimization on a
specific set of paths rather than on the top module or submodules, which reduces the
area overhead. You can apply these attributes in the RTL or in the XDC file. For more
information, including usage and restrictions, see the
Vivado
Design Suite User Guide: Synthesis (UG901).
The following figure shows 58 paths with five logic levels within the inst1/inst2 hierarchy constrained with the 600 MHz clock and 32 paths with only 1 logic level.
Vivado synthesis can rebalance the logic levels by moving the registers in the low logic level paths into the high logic level paths. In this example, you can add the following constraint to the synthesis XDC file to perform retiming on the inst1/inst2 hierarchy:
set_property BLOCK_SYNTH.RETIMING 1 [get_cells inst1/inst2]After rerunning synthesis with the same global settings and the updated XDC file,
you can run regular timing analysis on the inst1/inst2 timing paths or rerun
the report_design_analysis command to
verify that the longest paths have fewer logic levels, as shown in the
following figure. The critical path is now REG0 > 3 LUTs > REG2
(backward retimed), and the path from REG2 to REG4 has 3 logic levels.
