Who can tell me how to compare the external capacitor value with a reference value? If the cap is large than the reference, do some operation. And the cap is less than the reference, disable the operation. Such as the RC pin in bq2000.
how about charging the cap at some voltage value and compare the discharge through some big R.
iit has exponential behaviour you would be able to predict if the cap is larger or not.
Just a simple voltage comparator would work.