I2C driving of this RTC chip from a PIC microcontroller 16F877. Display via a graphical LCD. Character generation with an I2C EEPROM. Time and date are retained up to 10 years with a small 3V lithium battery.
http://www.techdesign.be/projects/007/007.htm
This circuit provides a digital square wave that can be viewed directly or used to drive other circuits. It used the CMOS 4047 Low-Power Monostable/Astable Multivibrator.
http://members.tripod.com/~baec/articles/clock.htm
using AT89C2051
http://chaokhun.kmitl.ac.th/~kswichit/clock/clock.htm
voltage controller oscillator that was designed as a wide range oscillator to generate clock pulses for a stepper motor drive system, frequency could be varied over 1,000,000:1
http://www.4qdtec.com/pco.html
optosolated circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit, provides 5000V isolation from line
http://www.imagineeringezine.com/PDF-FILES/60hzclk.pdf