The Summary by Clock Pair section shows how many clock domain crossing (CDC) paths exist between two clocks and identifies the most critical CDC issue. This section helps you quickly assess the most severe CDC violations across clock pairs.
An example is shown in the following figure.
Figure 1. Summary by Clock Pair Section
The table includes the following columns:
- Severity
- Shows the most critical severity level among all CDC paths
between the listed clocks. Values include:
- Info
- Warning
- Critical
- Source Clock
- Displays the name of the source clock for the CDC path.
- Destination Clock
- Displays the name of the destination clock for the CDC path.
- CDC Type
- Describes the relationship between the two clocks and the
dominant timing exception, if any. Types include:
- Safely Timed
- Clocks are synchronous and all CDC paths are accurately timed.
- User Ignored
- All CDC paths are covered by
set_false_pathorset_clock_groups. - No Common Primary Clock
- Clocks are asynchronous, and at least one CDC path exists between clocks without a common primary clock.
- No Common Period
- Clocks are asynchronous and do not share a common period. For the definition of clocks with no common period, refer to Timing Analysis Key Concepts.
- No Common Phase
- Clocks are asynchronous with no known phase relationship.
- Exceptions
- The timing exceptions applied to the CDC (if any) are:
- None
- No
set_clock_groups,set_false_path, orset_max_delay -datapath_onlyexceptions exist for the CDC. - Asynch Clock Groups
-
set_clock_groups -asynchronousis applied on the CDC. - Exclusive Clock Groups
-
set_clock_groups -exclusiveis applied on the CDC. - False Path
-
set_false_pathis applied between the clocks or on all CDC paths. - Max Delay Datapath Only
-
set_max_delay -datapath_onlyis applied to all paths. - Partial Exceptions
- A combination of
set_false_pathandset_max_delay -datapath_onlyis applied, and at least one CDC path is still timed.
- Endpoints
- Reports the total number of CDC path endpoints. Each endpoint is a sequential cell input pin. A flip-flop (FD) cell can count as multiple endpoints depending on connectivity to D, CE, or control pins (such as SET, RESET, CLEAR, or PRESET). Some topologies report only the first pin, even when there are several paths crossing the clock domain boundary to reach the CDC structure.
- Safe
- Indicates the number of CDC path endpoints considered safe.
Safe endpoints meet one of the following conditions:
- Path uses asynchronous clocks and known safe CDC structures
- Path uses synchronous clocks with safe structures and timing exceptions
- Path uses synchronous clocks without exceptions and is safely timed
- Path synchronized with
HARD_SYNCmacro
- Unsafe
- Reports the number of CDC path endpoints identified as unsafe.
Unsafe topologies include:
- CDC-10: Combinatorial logic in fanin
- CDC-11: Fanout before synchronizer
- CDC-12: Multiple clocks in fanin
- CDC-13: CDC on a non-FD primitive
- Unknown
- Shows the number of endpoints that use unknown or unrecognized
CDC structures. These include:
- CDC-1: Single-bit CDC without known synchronization
- CDC-4: Multi-bit CDC without known synchronization
- CDC-7: Asynchronous reset without synchronization
- No ASYNC_REG
- Indicates how many synchronizers are missing the
ASYNC_REGproperty on one or both of the first two flip-flops in the synchronizer chain.