Well, it seems strange that zero crossing being done directly using microcontroller with the help of two resistors only. AVR application note 182 verifies that zero crossing of 220 AC is possible using two resistors only. Here is how it looks like:
There is no rocket science in the above arrangement. 220AC through 1M ohm resistor, being fed directly to the AVR microcontroller will not do any harm to it. This is because the microcontrollers have internal clamping diodes that limits the high input voltage to microcontrollers operating voltage. The resistors here are used to convert a high voltage AC sinusoid to low voltage square wave. Thereafter using interrupts of the microcontroller, one can easily do the zero crossing detection.
However, there are some issues regarding the above arrangement. First of all EMI (Electromagnetic Compatibility) issue can arise. This is due to the fact that there is no isolation between the Hot Line and the microcontroller and the induced noise may affect the functionality both in the inter or intra system. Secondly, the resistor connected with AC lines act as a RC filter and thereby generating a minute phase delay. But this minute phase delay can be neglected in most of the applications. This method for the zero crossing detection is still considered very efficient.
Care should also be taken that high AC voltage may not damage the other components. There have been cases, I personally know, that people have burned out their microcontroller just because of little carelessness.
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