Guzunty Pi 8p8i core PWM clock frequency selection

 Now I am replacing the RPi's GPCLK0 signal, set by gz_8p8i.c to 5 MHz, by an external clock.

This external clock is generated by the 10MHz quartz oscillator, divided down my 2 CD4020 14 stage ripple binary counter to a low frequency of order 1 Hz.  I found using 2.44 KHz a good choice for input to Guzunty Pi, because the PWM output would be around 75Hz.

For servos such as TowerPro MG995, the minimum cycle time is 20mS, which means 1000/20 = 50Hz.  So 75 Hz should be fast enough.

Next step is to write a Python function to set the PWM duty cycle.

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