Imagine that you want to measure 4 "RC" PWM signals, standard being 
about 1mS to 2mS at 50 Hz (IIRC)
You want 8 bit accuracy so that leaves you with 1mS/256 = 4uS resolution.
There are a few schemes I can think of to achieve this, but I don't 
think any are particularly good:

1. Polling - have a running timer and poll input pins. Resolution will 
suffer so you will probably have to average several measurements to 
(hopefully) get a good result. You won't really be able to do anything 
else with your PIC.
2. Interrupt on change - your interrupt code runs and checks the pins 
when one changes. Chaos results when pins change within uS of each other 
and the interrupt code is still executing.
3. Capture - probably the correct way to do this (pin interrupt stops 
timer). The problem is that I can't imagine there being a chip with 4 
such CCP inputs

How would you do it without a lot of external logic?

De-mux into a capture pin maybe?
I'd lose f/n resolution: 50/4 = 12.5 samples per second, maybe that's 
good enough. Then I'd want to add edge triggering to ensure that I don't 
capture half-pulses, meaning more external logic.
-
Martin
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