"...by using a prescaler by 256, the maximum signal to measure is 5078125 Hz so a pulse every 19.7 microseconds."

Stephane, that's 5MHz or 197 ns!

A 'standard' 8051 can count frequencies up to 1/24th of the oscillator frequency. For your 668 that might be 1/12th in 6-clock mode (not sure about that) but even then, the max. counting frequency will be 2MHz with a 24MHz oscillator.

Also, don't confuse accuracy with resolution. As Peter already said, the accuracy is determined by the reference oscillator. For a 24-bit counter, this oscillator should be accurate or at least stable to within 0.06 ppm!

I agree with Erik; using a CPLD would be my first choice also.

Best regards,
Rob.