Frequency Counter: Oscillator Schematic
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.