Rob,

I've read again the p89c668 datasheet and I didn't seen the 2 MHz operation on the PCA capture pins you were speaking about ...
Where did you found this information ?

I've read that the internal frequency for PCA could be set to Fosc/6 or Fosc/2 which I suppose means Fcrystal/6 or /2...

With a 2 MHz operation, it could allow to measure a frequency up to 500MHz with an only /256 prescaler and up to 1.2Ghz with a /128 prescaler and a divider by 4...

The only thing is to build a 32 bits timer to keep a good resolution and all is done. It seems possible to build a such timer using PCA and a timer....

The oscillator has to be accurate so an only crystal won't be enough. I should take a thermal compensated oscillator or to build an accurate and adjustable oscillator which shouldn't be so hard...

By this way, it would be possible to build a frequency meter with very low components and with good features....
To test....

Regards
Stephane