Intel® Quartus® Prime Pro Edition User Guide: Timing Analyzer

ID 683243
Date 9/26/2022
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2.2.10. Time Borrowing

Time borrowing can improve performance by enabling the path ending at a time-borrowing flip-flop or latch to "borrow" time from the next path in the register pipeline. The borrowed time subtracts from the next path, resulting in the same cumulative timing. In this way, time borrowing can shift slack to more critical parts of the design. Without time borrowing, the Timing Analyzer analyzes each path independently and normally assumes exactly one clock cycle for each transfer.
Figure 34. Time Borrowing Example

Some of the flip-flops in Intel® Stratix® 10 and Intel® Arria® 10 devices allow time borrowing. The exact size of the available time borrow window depends on hardware settings. The Fitter (Finalize) stage automatically configures the appropriate borrow window for each time-borrowing flip-flop, based on hardware restrictions and the available hold slack.1

Figure 35. Time Borrowing with Various Slack Conditions and Borrow Values

Intel FPGA devices generally support only a few borrow window sizes. For example, Intel® Stratix® 10 devices support narrow, medium, and wide. Typically, groups of several flip-flops must share the same setting. The actual borrowed amount is completely flexible within a given borrow window. The Timing Analyzer calculates the borrowed amount separately for each operating condition, clock, and signal rise and fall edge. Selecting a wider borrow window reduces hold slack. The Compiler only selects wider settings if hold slack allows. Furthermore, if the Compiler determines that a narrower window is sufficient for a given group of registers (based on the optimal time borrowing solution), the Compiler uses the narrower window, even if there is sufficient hold slack for a wider window.

For a given borrow window size, the exact size of the borrow window may depend on the register input (for example, d or sclr), the edge of the incoming signal (rising or falling), the device speed grade, and operating conditions.

You can enable automatic implementation of time borrowing without making any RTL changes. Once enabled, the Fitter automatically configures the window size. The Fitter also determines the optimal time borrow amount within the available borrow window for any design registers that the Fitter places in time-borrowing flip-flops.

Proper timing analysis of designs that contain level-sensitive latches typically requires time borrowing. However, the automatic Fitter time borrowing optimizations do not apply for level-sensitive latches, as Time Borrowing with Latches describes in detail.

1 In Partial Reconfiguration designs, additional restrictions may apply to time-borrowing window size.