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Frequency divider
This is the circuit that transforms exact 10MHz signal that was made with the PLL oscillator to the frequency 50Hz that is used in the digital clock main board.
The decade counter is used to count down the frequency. In SN74HC390 that is using this time 2 pieces of 2 binary counters and 2 pieces of 5 binary counters are included in a package. Therefore the frequency can be count down to 1/100 with a IC by using each counter for a series.
3 ICs are used because it needs to divide 1/200,000 to count down 10MHz to 50Hz.
