NSS-30013: Design Contains Ripple Clock Structures

Ripple clock structures are structures where the outputs of two or more registers in a cascade each directly drives the input clock port of the following register in the cascade.

The following image shows an example of a ripple clock structure:

Figure 1. Ripple Clock Structure

Each stage of a ripple clock structure causes phase delay, which accumulates and results in large skews in the structure's output signal. The large skew can cause timing closure problems when you use the ripple clock structure as a clock signal for other circuits. Each stage of a ripple clock structure also causes a new clock domain to be defined. The additional clock domains make timing analysis of the design more complex and time-consuming.

Recommendation

Avoid including ripple clock structures in your design. Ripple clock structures are often used to make counters out of the smallest amount of logic possible. However, in all Intel devices supported by the Intel® Quartus® Prime Pro Edition software, using a ripple clock structure to reduce the amount of logic used for a counter is unnecessary because the device allows you to construct a counter using one logic element per counter bit.

Severity

Medium

Stage

Analysis and Elaboration

Device Family

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