thanks for responding, but my confusion starts with the MCU data sheet. There is a prescaler that at one point in the data sheet says the internal 8Mhz oscillator divides by 8 to get the system clock. I have found no mention in any of the Arduino literature to say whether the 16Mhz clock on the Mega is divided by the prescaler or not. What I need to know is, is this 16MHz clock the one that feeds the timer/counters?
I can of course just set it going and use pulseln() to measure the frequency. This is what I will do when I get some time to experiment, but I wondered if this knowledge was documented anywhere.