Frequency Counter: Oscillator Schematic
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The oscillator provides a 1MHz and a 1khz clock. The 1MHz clock is used as
the clock for the microcontroller and as timebase for millisecond
measurement. I use a 2MHz and divide it by 2 in order to guarantee a
50% duty cycle. Then three halve HC390 are used as divider by 1000 which
can be externally reset. This is necessary to make sure the millisecond
timer starts running at the time it is needed. AC04 are used as
drivers/buffers. J1 allows the use of an external clock.