CDC

1
CDC
Clock Domain Crossing
2
Outline

• Introduction
• Basic terminology
• Types of synchronizers
• CDC protocols
• CDC verification
• Summary

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Basic Terminology

• Clock Domain

- Single clock domain
- Multiple clocks domain
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Basic Terminology

• Clock Domain

divCLK
CLK
CLK
divCLK
Single clock domain
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Basic Terminology

• Clock Domain

CLK A
CLK B
CLK A
CLK B
Multiple clocks domain
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Basic Terminology

• Clock Domain Crossing Paths

D
DA
DB
CLK A
CLK B
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Basic Terminology

• Metastability

D
DA
DB
CLK A
CLK B
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Basic Terminology

• Synchronizers

Circuits that conditions CDC signals in order to
reduce the probability of metastability
DA
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Types Of Synchronizers

• Basic Synchronizer (Level Signal)

• Edge-detecting Synchronizer

CLK B
XOR

• Pulse Synchronizer

CLK B
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CDC Protocols

• Why do we need protocols ?

In may applications, simple signals arent
the only information crossing clock
domains. Data, address and control buses
also travel together across domains.
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CDC Protocols

• Handshake

TX
RX
Sync
rx_v
tx_v
Sync
tx_d
rx_d
tx_datat
rx_datat
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CDC Protocols

• FIFO

TX
RX
FIFO
r_datat
w_datat
wr
rd
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CDC Verification

• Do i have synchronizers on all
• CDC signals?

• Do my CDC protocol prevent corruption
• of data across clock domains?

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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.

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Summary
Traditional verification techniques for CDCs
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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