1 CDC Clock Domain Crossing

2 Outline Introduction Introduction Basic terminology Basic terminology Types of synchronizers Types of synchronizers CDC protocols CDC protocols CDC verification CDC verification Summary Summary

3 Basic Terminology Clock Domain Clock Domain - Single clock domain - Single clock domain - Multiple clocks domain - Multiple clocks domain

4 Basic Terminology Single clock domain Clock Domain Clock Domain CLK divCLK CLK divCLK

5 Basic Terminology Clock Domain Clock Domain Multiple clocks domain CLK B CLK A CLK B

6 Basic Terminology Clock Domain Crossing Paths Clock Domain Crossing Paths CLK A CLK B DDADB

7 Basic Terminology Metastability Metastability CLK A CLK B DDADB CLK A DA CLK B DB

8 Basic Terminology Synchronizers Synchronizers Circuits that conditions CDC signals in order to reduce the probability of metastability CLK A CLK B D DA

9 Types Of Synchronizers Basic Synchronizer (Level Signal) Basic Synchronizer (Level Signal) Edge-detecting Synchronizer Edge-detecting Synchronizer CLK B Pulse Synchronizer Pulse Synchronizer CLK B XOR

10 CDC Protocols Why do we need protocols ? Why do we need protocols ? In may applications, simple signals aren’t the In may applications, simple signals aren’t the only information crossing clock domains. only information crossing clock domains. Data, address and control buses also travel Data, address and control buses also travel together across domains. together across domains.

11 CDC Protocols Handshake Handshake TX RX Sync Sync tx_datat rx_datat rx_vtx_v tx_d rx_d

12 CDC Protocols FIFO FIFO TX RX w_datat r_datat FIFO wrrd

13 CDC Verification Do i have synchronizers on all Do i have synchronizers on all CDC signals? CDC signals? Do my CDC protocol prevent corruption Do my CDC protocol prevent corruption of data across clock domains? of data across clock domains?

14 CDC Verification 1.Assertion Based Verification. 2.Netlist Analysis. 3.User Specification of Design Intent. 4.Verifying CDC Monitors Using Simulation. 5.Verifying CDC Assertion Using Formal Verification.

15 Summary Traditional verification techniques for CDC’s fail to address the critical issue of the correctness of the CDC protocols within the design. If these protocols go unchecked, there is a significant risk that designs will contain errors that can only be detected in post-silicon testing….

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