Sometimes things come out right....
Off at the client site again this afternoon, checking that I'm looking at the right polarity for all the signals that come into my controller from their box. Comes time to test the excessive-pulse-width signal (a rough check in hardware, as my precision pulse-width checking depends on the pulse actually having a trailing edge).
Oops. It doesn't latch. An overly-wide input pulse generates about a 300 microsecond error pulse, which my polling doesn't often catch.
But wait! I routed that signal in on P0.15 for a reason! P0.15 can also be configured as EINT2, for prompt response and edge-triggeredness!
Now, how do I go about that... and (eep!) is the polarity programmable?
Aha! Yes, the pin configuration register is used to turn it into EINT2, and the EXTMODE and EXTPOLAR registers of the LPC2134 (as this happens to be) are used to configure edge vs. level, and the polarity. All's well!