[time-nuts] Seeking advice on trigger functionality for frequency counter/timer design

[Erik Kaashoek]

Now the basic functionality of the DIY counter is performing well it is 
time to look at what other functionality makes sense
One area that is very new to me is the external triggering functionality.
I've studied the Keysight 53230A user manual and I thing I understand 
its trigger functionality but replicating everything is rather a lot of 
work and may not fit with the currently selected HW.
In particular the time relation  between the external trigger and the 
opening/closing of the gate is a difficult design decision.
If the time relation is weak, e.g time accuracy of trigger input 
happening and actual actions in the timer have an uncertainty of micro 
seconds or even milliseconds, it may be possible to implement all the 
relations between trigger input and gate manipulation in SW.
If the time relation is strong, such as when the gate should be opened 
nanoseconds after the trigger input, the trigger logic has to be 
implemented in HW and may not fit in the selected CPLD.
So my rather generic question to the esteemed members of this group is: 
What minimum functionality of external triggering would make sense in a 
low cost counter/timer?
What would be meaningful use cases for an external trigger input and 
what would be the timing constraints?
Erik.