HMI430/700: NVEM3, RTC RAM

NVEM3 is 20 bytes of RTC battery backed RAM.  It's fast and has no limits to the number of write cycles.  This makes it better than some other NVEM pages that usually have a limited number of write cycles.

However, this RAM will only be retained when power is off providing you have a RTC battery fitted!  You can still use it without a RTC battery, but the contents will be lost when power is turned off.