CDC-50004: MUX-type CDC Transfer with Insufficient Constraints

Description

Violations of this rule identify a CDC transfer controlled by a MUX or a MUX-like topology with insufficient timing constraints. Without proper constraints, all bits of such a topology may not latch on the same clock cycle.

Figure 1. Topologies checked by CDC-50004.. MUX-type transfers include topologies where one input of the MUX is tied to VCC or ground, as well as transfers controlled by synchronous clear, as the following figure shows.

Recommendation

Apply an instance assignment of Synchronization Register Chain Length = 1 on the head of a MUX-type CDC to prevent downstream registers from being treated as a synchronizer chain. Next, if the transfer forms a multi-bit bus, apply a set_max_skew constraint on the bits of the 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, for all CDC widths, apply a set_data_delay constraint on the bits of the transfer 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]

Severity

High

Tags

Tag Description
cdc-bus Design rule checks related to topologies that use a bus to transfer multiple bits of data between clock domains at once.

Device Family

  • Intel®Cyclone® 10 GX
  • Intel®Arria® 10
  • Intel®Stratix® 10
  • Intel®Agilex™