[time-nuts] 60Hz mains clocking in computers

Bill Hawkins bill at iaxs.net
Sun Dec 13 17:39:39 UTC 2009


Industrial process control requires that event time stamps be close to
the correct social (wall clock) time, for correlation with events that
were not digitized. Computers at the heart of these control systems
were required to run on DC from batteries, and so the real-time clock
was derived from a local crystal. (Make that micro-computers. Work was
still done with line-frequency-aware computers.)

The system that I helped design had a clock rate adjustment for social
time, expressed as one byte. The user was able to adjust the clock
rate for the desired accuracy. A clever algorithm minimized the amount
of code added to the clock interrupt routine.

But, I have only studied one of the elephant's legs.

Bill Hawkins




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