CDC

1. CDC Clock Domain Crossing

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

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

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

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

6. Basic Terminology • Clock Domain Crossing Paths D DA DB CLK A CLK B

7. CLK A DA CLK B DB Basic Terminology • Metastability D DA DB CLK A CLK B

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

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

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

11. CDC Protocols • Handshake TX RX Sync tx_v rx_v tx_d Sync rx_d tx_datat rx_datat

12. CDC Protocols • FIFO TX RX FIFO r_datat w_datat wr rd

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

14. CDC Verification • Assertion Based Verification. • Netlist Analysis. • User Specification of Design Intent. • Verifying CDC Monitors Using Simulation. • 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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