NSS-30016: Design Contains SR Latches

Description

SR latches are structures where two two-input NOR gates or two-input NAND gates (which the Compiler implements in logic cells) are cross-coupled using combinational loops that drive the output of one gate to an input of the other gate.

An SR latch can cause glitches and ambiguous timing in a design, which makes timing analysis of the design more difficult. In addition, an SR latch can cause significant stability and reliability problems in a design because the behavior of the combinational loops in the latch often depends on the relative propagation delays of the combinational loop's logic, causing the combinational loop to behave differently under different operation conditions.

Figure 1. SR Latch

The Design Assistant also generates this rule for SR latches that are part of more sophisticated latches that the Design Assistant cannot identify.

Recommendation

Avoid including SR latches in your design.

Severity

High

Stage

Analysis and Elaboration

Device Family

  • Intel® Arria® 10
  • Intel® Cyclone® 10 GX