Hi,

I am currently designing a system around the C8051F120 to measure and display voltage, current, frequency, and power factor from a 120V 3A ~60Hz source.
My code is currently written in C.

I have voltage and current working and is accurately displaying on my LCD to the 1000th place. I am using the on board 12-bit ADC to do the A2D conversions.
Allis fine until I got to Power Factor measurements/calculations. I know how to use a comparator (after much testing with the LM339). I am able to use it to convert my 5V analog sine wave to a square wave (Although with much noise) Still need to add in filters and signal conditioning. but how do I use it to detect a zero crossing?

My problem now lies in using the square wave to determine my zero crossing. I've seen circuits use a, xor gate? Not sure how to apply this concept to my design with a LM339.
How "exactly" should I use the zero crossing to calc. PF?

Software wise...I have somewhat of idea on how to use zero crossing. By using a switch and a while loop to check for a high value from the storage cap. Then perform operations and set back to the loop. Any direction on where/how to approach this, or an C algorithm would be GREATLY appreciated. Also, if anyone has an alternative to what I am doing, please let me know, I am willing to learn from you!

Thanks in advance for any help.

Leon
..novice in electrical design...