All code is carefully timed to run for one second, during it it counts the rising edges on the EAR port(bit6 of port 254). HL counts the pulses, BC is a dec counter responsible for the one second total time.

Initially it did not work, then I look at the harlequin schematic and saw what data line the EAR was connected to.

This idea dates back to the early 90's when I bought a ZX Spectrum +3.

I had a manual with the assembler codes and timings, and I kind did something like this, I had no signal generator(still don't that's a online signal generator) so who knows :)

Back then there was no internet, I must been 16 or 17 years old.

I bought the harlequin and soldered (1093 solder joints) only so I could recreate this code. :)

0.3% error is pretty good


The last picture is the full code in BASIC with the machine code loader.

The assembly is here:
https://github.com/titojff/Z80-Frequency-counter-ZX-spectrum/tree/main

Also included the .tzx file with all the code.