I would like a couple of examples of any products that use the power
line frequency as a CRITICAL reference as I know of none.


Designer talk...
http://www.electronicspoint.com/long...on-t70834.html

Then I've read tons of stuff like this...
"One way to provide an RTC solution with a fairly high degree
of accuracy over-temperature is to use an external, very
accurate clock source. The AC power in the U.S. and
Europe is such a source, with either 60Hz or 50Hz frequency
that is extremely stable over long time periods as well as
over- temperature."...
http://www.intersil.com/data/an/an1342.pdf

Another...
"The HSI frequency is not measured directly, but it is estimated from
the number of HSI clock
pulses counted using a timer, and compared to an ideal value: 8 000
000 Hz. To do so, a
very accurate reference frequency must be available such as the RTC/64
signal provided by
the external 32 kHz crystal or the 50 Hz/60 Hz of the mains"
See page 6 and 7...
http://www.st.com/internet/com/TECHN...CD00221839.pdf