Hi Alain,
Just want to solve the puzzle.
So PIC and PICBASIC PRO or Assembler do not helpful in present case.
Main point of solving the problem is to make 8 bit ADC external.
So 8bit equal 256. Single cycle of conversation is 500nSec / 256 is equal 1.95 nSEC.
Crystal clock has to be 512 MHz then divided by ~25 will get 20Mhz that will feed PIC.
From this point of you are right and I agree with you.
If we decide to take 4bits ADC conversations or 16 steps 500nSec/16 is equal 32 MHz.
It is close to final.
If external ADC has build in comparator and RC circuit conversations can be done in 200nSec.External clock is 80 MHz.
So we have extra 300nSec + 500nSec is equal 800nSEc that means we can use
four instruction for reading from 4bits PORT and storing in external 4bits PORT storage like RAM data by 40Mhz PIC16F.
Where am I wrong?
Best Regards