Understanding Clock Domain Crossing Issues

By Saurabh Verma, Ashima S. Dabare, Atrenta
December 24, 2007 -- edadesignline.com

Introduction

SoCs are becoming more complex these days. A lot of functionality is being added to chips and data is frequently transferred from one clock domain to another. Hence, clock domain crossing verification has become one of the major verification challenges in deep submicron designs.

A clock domain crossing occurs whenever data is transferred from a flop driven by one clock to a flop driven by another clock.

1. Clock domain crossing.

In Figure 1, signal A is launched by the C1 clock domain and needs to be captured properly by the C2 clock domain. Depending on the relationship between the two clocks, there could be different types of problems in transferring data from the source clock to the destination clock. Along with that, the solutions to those problems can also be different.

Traditional methods like simulation and static timing analysis alone are not sufficient to verify that the data is transferred consistently and reliably across clock domains. Hence, new verification methodologies are required, but before devising a new methodology it is important to understand the issues related to clock domain crossings properly. Different types of clock domain crossings are discussed here along with the possible issues encountered in each one of them and their solutions. A new verification methodology is then proposed which will ensure that data is transferred correctly across clock domains.

In all the subsequent sections, the signal names shown in Figure 1 are directly used. For example, C1 and C2 imply the source and destination clocks respectively. Similarly A and B are used as source and destination flop outputs respectively. Also, the source and destination flops are assumed to be positive edge triggered.

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