Timequest - specify different output delays for different paths

The fact that you have two internal Tco requirements should not mean you need two output delays. If the pin is the same, then the external delays are the same, and therefore, you only need to worry about a single output delay. Or are the output delay changing based on which register it is coming from?

That said, you should be able to use set_max_delay to define the delays on these two different paths.

Hope this helps.

--- Quote Start ---

The fact that you have two internal Tco requirements should not mean you need two output delays. If the pin is the same, then the external delays are the same, and therefore, you only need to worry about a single output delay. Or are the output delay changing based on which register it is coming from?.

--- Quote End ---

I'm designing to a specification so actual external delays don't matter. I just need to express what the specification says.

--- Quote Start ---

That said, you should be able to use set_max_delay to define the delays on these two different paths.

--- Quote End ---

Thanks, it looks like that may do what i need. I'll try it.

set_max_delay is the correct override. I believe the original PCI worked like that, whereby all data was sent within the clock period, but the OE timing was a requirement in the middle of the clock window, so that all drivers were off before somebody new starts driving the bus.

Remember that set_max_delay and set_output_delay work together. For example, if you do a "set_output_delay -clock ext_10ns_clk -max 3 [get_ports data_out*]" then you're saying data_out* drives an external register that is clocked by ext_10ns_clk and the external delay to get there is 3ns. There will be a setup relationship between the internal clock driving your output register and this external clock. Most likely both clocks are the same frequency, so let's say it's a 10ns setup relationship. When you apply a "set_max_delay -from [get_keepers oe_reg*] 7", you're overriding the setup relationship from the oe_reg to the external register, so instead of 10ns it is now 7ns, but the external delay is still there. So, in this example, the set_max_delay makes the setup relationship become 7ns and 3ns of that will be used externally, so the oe has to get its data out in 4ns, basically like a 4ns Tco. You should see in the report_timing a value called oExt that shows this external delay.

I mention this because users often think of set_max_delay as a complete override, and so it acts like a Tco. That's true if there is no external delay(and no phase-shift or falling edge, but that's another story). But if there is an external delay, that is used in set_max_delay's calculation.