126.96.36.199. Time Borrowing Limitations
- Any register that is the source of a cross-clock transfer
- Any register that is the source of a set_max_skew or set_max_delay assignment
- Any register in a clock domain with one or more level-sensitive latch
Furthermore, registers that are destinations of cross-clock transfers, set_max_skew or set_max_delay constraints do not have borrowing values that are optimized for such transfers (but may still have non-zero borrowing from other transfers).
If any such registers are on the critical timing path, you can possibly report better performance by enabling Dynamic time borrowing mode, which reports time borrowing for all borrow-capable registers. Dynamic time borrowing mode can then provide a more accurate (less pessimistic) analysis, but only at a specific set of clock frequencies that you specify in the .sdc.
To view time borrowing results for such registers:
- Set the clock frequency in the .sdc file to a value higher than the clock frequency that Fmax Summary reports.
- Reset your design and read the SDC file again in the Timing Analyzer.
- Run the update_timing_netlist -dynamic_borrow command.
- View the results in Slack Summary reports (Reports > Slack to determine if timing passes. The Fmax Summary report does not reflect any gains from dynamic borrowing.
Time borrowing is similar to the beneficial clock skew technique, whereby you delay the clock to a given register, giving more time to the incoming paths at the expense of the outgoing paths. However, time borrowing and beneficial clock skew have the following important differences:
- Time borrowing offers more flexibility than clock skew in distributing setup slack. Skewing is typically limited to fixed increments. You can use borrowing to shift any amount of slack (however small) from one side of the register to the other, as long as the amounts fit within the available borrow window. Furthermore, borrow amounts calculate separately for each operating condition, making it possible to shift the optimum amount of slack for each operating condition. This type of shifting is not possible with skewing.
- You can use beneficial clock skew to increase Hold slack on outgoing paths from the register where the clock is skewed. Time borrowing does not offer this benefit.