If you wanted to use a counter to divide your external signal by 2 you would have to do something like this. However... the counter will only issue a short pulse on every second edge of the external signal, so your requirement to use the falling edge of this signal is probably going to make this not an ideal solution. Making the counter toggle instead of pulse is probably closer to what you want, but the minimum number of ticks between transitions is 2 (so 2 ticks high, 2 ticks low, which would give a minimum divide-down of 4).
Assuming you want the output signal to be 50% duty cycle at half of the frequency of the input signal, you could configure a digital output with a waveform of [1,0,1,0,1,0...] using the external signal as your sample clock. You would have to wire the digital output line back into a PFI line in order to route it to your counter outputs.
Best Regards,
John Passiak
