A very good project Marcel..
Just a suggestion, all the aftermarket add on readouts seem to ignore the megacycle frequency part, when this is very easy to? include.
The Mcs counter only needs a simple prescaler , for example,? ?CMOS HTC7490 divide by 10 connected tp TP101 via a simple buffer amplifier. You already have unused gates in the 7414, that can be used, just? change this to pin compatible? HCT version because of speed This will give? an output frequency of 5.5 mHz to 7 mHz easily counted by the Arduino. Its only the first two digits are required and sent to the left hand display. The second count then takes care of the kHz as you have already done..
So, count up the 5.5 to 7 mHZ prescaler output, subtract the 5.5 mHZ? IF offset, latch and display the two? most significant? digits, then count and display the four kHZ digits, as already done.. By monitoring the "lock"? LED, the Arduino could blank or flash? the mHZ display to indicate mistuning .
?To add this to your existing design would involve only one more IC package, and a slight rework of your PCB, and some extra code.
