Understanding the Clock Domain Crossings Report Rules - 2021.2 English

Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906)

Document ID
UG906
Release Date
2021-10-27
Version
2021.2 English

Report CDC tries to match each CDC path to a known CDC topology. Each CDC topology is associated with one or several CDC rules, as presented in Table 1. Note that you cannot modify the severity of the rules as with DRCs and Messages. Simplified schematics and descriptions of the CDC Topologies being detected are included in Simplified Schematics of the CDC Topologies.

The CDC topologies are analyzed based on some precedence rules. Table 2 shows the CDC rules ordered from the highest to the lowest precedence. By default, only one CDC violation is reported at most per endpoint and if multiple violations exist on a particular endpoint, the CDC rule with the highest precedence is reported and masks any lower precedence CDC violation. For example, since CDC-15 has a higher precedence than CDC-10, a safe CDC-15 detected on a register masks an unsafe CDC-10 on the pin D of the same register.

Note: The default behavior can be overridden with the following command line option:

-all_checks_per_endpoint

This option forces the tool to report all the Info, Warning, and Critical checks that apply on the endpoints, regardless of the rules of precedence. However, unsafe rules on a register are not reported if at least one safe rule on the same register is detected.

Table 1. CDC Rules and Description
CDC Topology CDC Rule Severity Description
Single-bit CDC CDC-1 Critical A single-bit CDC path is not synchronized or has unknown CDC circuitry.
CDC-2 Warning A single-bit CDC path is synchronized with a 2+ stage synchronizer but the ASYNC_REG property is missing on all or some of the synchronizer flip-flops.
CDC-3 Info A single-bit CDC path is synchronized with a 2+ stage synchronizer and the ASYNC_REG property is present.
Multi-bit CDC CDC-4 Critical A multi-bit bus CDC path is not synchronized or has unknown CDC circuitry.
CDC-5 Warning A multi-bit bus CDC path is synchronized with a 2+ stage synchronizer but the ASYNC_REG property is missing on all or some of the synchronizer flip-flops.
CDC-6 Warning A multi-bit bus CDC path is synchronized with a 2+ stage synchronizer and the ASYNC_REG property is present.
Asynchronous Reset CDC-7 Critical An asynchronous signal (clear or preset) is not synchronized or has unknown CDC circuitry.
CDC-8 Warning An asynchronous signal (clear or preset) is synchronized but the ASYNC_REG property is missing on all or some of the synchronizer flip-flops.
CDC-9 Info An asynchronous reset is synchronized and the ASYNC_REG property is present.
Combinatorial Logic CDC-10 Critical Combinatorial logic has been detected in the fanin of a synchronization circuit.
Fanout CDC-11 Critical A fanout has been detected before a synchronization circuit.
Multi-Clock Fanin CDC-12 Critical Data from multiple clocks are found in the fanin of a synchronization circuit.
non-FD primitive CDC-13 Critical CDC detected on a non-FD primitive.
CE-controlled CDC CDC-15 Warning Clock Enable controlled CDC.
Mux-controlled CDC CDC-16 Warning Multiplexer controlled CDC.
Mux Data Hold CDC CDC-17 Warning Multiplexer data holding CDC.
HARD_SYNC primitive CDC-18 Info A signal is synchronized with a HARD_SYNC primitive.
LUTRAM-to-FD CDC CDC-26 Warning LUTRAM read/write potential collision.
Table 2. CDC Rules and Precedence (Highest to Lowest)
CDC Topology CDC Rule
HARD_SYNC primitive CDC-18
Non-FD primitive CDC-13
Mux Data Hold CDC CDC-17
Mux-controlled CDC CDC-16
CE-controlled CDC CDC-15
LUTRAM-to-FD CDC CDC-26
Asynchronous Reset CDC-7
Single-bit CDC not synchronized CDC-1
Multi-bit CDC not synchronized CDC-4
Multi-Clock Fanin CDC-12
Combinatorial Logic CDC-10
Fanout CDC-11
Asynchronous Reset synchronized with property CDC-9
Single-bit CDC synchronized with property CDC-3
Multi-bit CDC synchronized with property CDC-6
Asynchronous Reset synchronized without property CDC-8
Single-bit CDC synchronized without property CDC-2
Multi-bit CDC synchronized without property CDC-5