A far better way to measure the clocking rate of your processor is to use a circuit that picks the 50/60 Hz power line frequency off the input power source and gives that to a port pin or one of the external interrupt lines.

One way this can be done very easily is to power your device from an AC style wall wart. For powering the microcontroller you would put a full wave bridge rectifier and filter capacitor in front of what ever regulator chip you chose to use such as 7805. With the AC available at the input you can craft a simple coupling circuit to turn that into a 50/60Hz reference time base for the processor.

Here is a sample schematic of one way this could be incorporated into your microcontroller power supply.



The programming to contrast the operating speed of processor against the power line frequency reference is left as an exercise for the student.

Good Luck
Michael Karas