constrain CDCs with max_delay [vivado]

[jordens]

Apparently, the proper way to constrain some CDCs (Both AsyncResetSynchronizer and transfer via GreyCounters) should use a max_delay constraint and not a false path.

https://forums.xilinx.com/t5/Vivado-TCL-Community/How-to-set-timing-constraint-in-this-case/m-p/510771/highlight/true#M2049

How would one implement this?

A good entry to the rabbit hole: copied from http://www.colmryan.org/avrums-clock-domain-crossing-widsom.html#avrums-clock-domain-crossing-widsom :

Avrum is an active fellow on the Xilinx forums whenever clock domain crossing (CDC) issues crop up. By default, and in contrast to ISE, Vivado assumes all clocks are related. Thus, even with a proper synchronization circuit, Vivado needs to be explicitly told not to try and time these paths. Avrum does an excellent job of explaining the correct constraint relaxation to use and why not use what I call the "ostrich method" of just setting the clocks as asynchronous using set_clock_groups -asynchronous or set_false_path between all path between the clocks. This certainly is the easy way out but means you'll clobber all other more delicate constraints and could mask a real clock crossing problem where you've forgotten to synchronize. I've tried to collect his posts here for reference:

• post - constraints on a reset synchronizer and the one of the few places I've seen mentioned using a set_max_delay on a single-bit synchronizer chain
• post and post - clever tcl commands to use to find module and register instances
• post - more reset syncrhonization with a nice slide attached
• post - long discussion about the difference between single bit and bus synchronization
• post - long discussion of different types of CDC exceptions and why set_max_delay is almost always preferred
• post - not quite clock domain crossing but a clear explanation of IODELAYCTRL instantiation
• post - discussion of mean time before failure (MTBF) for synchronizer chain and references new Vivado tcl command report_synchronizer_mtbf for Ultrascale parts
• post - subtleties of distributed RAM which "is an odd beast - it is partly a synchronous element (the write) and partly a combinatorial element (the read)."

[jordens]

AsyncResetSynchronizer is now constrained accordingly. (#48)