Latch-Based Performance Optimization for FPGAs

Abstract

Abstract-We explore using pulsed latches for timing optimization -a first in the FPGA community. Pulsed latches are transparent latches driven by a clock with a non-standard (non-50%) duty cycle. We exploit existing functionality within commercial FPGA chips to implement latch-based optimizations that do not have the power or area drawbacks associated with other timing optimization approaches, such as clock skew and retiming. We propose an algorithm that iteratively replaces certain flip-flops in a logic design with latches for an improvement in circuit speed. Results show that much of the performance improvement achieved by using multiple skewed clocks can also be achieved using a single clock and latches. We also consider the impact of short delay paths (i.e. minimum delays), which can cause hold-time violations. Under conservative minimum delay assumptions, our latch-based optimization, operating on the routed design, provides a 5% performance improvement, on average, essentially for "free" (i.e. without any re-routing/delay padding). We show that short paths greatly hinder the ability of using latches for speed improvement, motivating further work to reduce their effects.

Architectural enhancements in Stratix V™

“…Pulse latches can also be used to perform small amounts of time borrowing between consecutive sequential paths. The use of latches in FPGAs to support time borrowing has also been explored in [15,18] with positive results in performance improvement. In contrast to selectively using pulse latches, we provided ubiquitous pulse latches in the Stratix V logic fabric.…”

Algebraic approach to time borrowing

“…Gunawardena noted that many of the results proved in [3,18] are instances of general theorems in max-plus algebra. Nearly twenty years on, max-plus methods do not appear to be widespread amongst the timing analysis community, with only a handful of papers in this area [15,16,20]. We believe that these methods have great potential to be exploited.…”