CDC-50004: CDC Bus Transfer through Logic with Insufficient Constraints

Description

Violations of this rule identify a CDC bus transfer through logic (such as a MUX-type synchronizer) with insufficient constraints. Without proper constraints, all bits of such a bus may not latch on the same clock cycle.

Recommendation

Apply a set_max_skew constraint on the bits of a CDC bus to ensure that all bits latch on the same clock cycle. The value of the skew constraint must be equal to or lower than either the source or destination clock period, whichever is lower. This can be accomplished with the following constraint:

set_max_skew -get_skew_value_from_clock_period min_clock_period -skew_value_multiplier $value_between_0_and_1 -from [get_registers $source_registers] -to [get_registers $destination_registers]

Also, apply a set_data_delay constraint on the bits of the bus to limit their allowable delay. The value of the data delay constraint must be equal to or lower than the destination clock period. This can be accomplished with the following constraint:

set_data_delay -get_value_from_clock_period dst_clock_period -value_multiplier $value_between_0_and_1 -from [get_registers $source_registers] -to [get_registers $destination_registers]

Do not apply a set_false_path constraint on the path, as it will override set_data_delay constraints.

Severity

High

Device Family