You posted: I think the +5V comes up on the port pins of the LPC921 before the +3.3V is fully stable on the Vdd pins of the LPC921.

Now you post: Actually I am not using much of 5V stuff, only driving a few leds and turning on some relays, and these are isolated from the micros pins with resistors and transistors.

Where is the connction?


In my opinion adding more devices to these "everything in a single chip" LPC devices defeats the very purpose of using these devices for small compact uncluttered reliable products (thats what I thought), in that case why use 3.3V devices and then add more components to make it reliable, might as well use 5V volt micros.
if you use the 3V3 chips because they are "everything in a single chip" and then have 1 or 2 things that need be 5V, the extras is not a lot. Often there has been cases where someone out of habit glued a whole bunch of 5V stuff that IS available in 3V3 onto a 3V3 uC and THAT is crazy.

Erik