[quote]
The peripheral expects good rise time and fall time
[/quote]
Do you mean you need a very short rise/fall time? Or you need a longer rise/fall time?
The RC value will determine the rise/fall time.

Since it is used as a trigger signal, especially, if you only need trigger voltage, you can use larger resistor/smaller capacitor combination. Anyway, it is determined by the specification/requirement for the trigger input signal.