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OK I have what SHOULD be a simple problem to solve, but it's driving me mad(der) by the day!! :?

I have an I and a Q circuit which derive two changing DC voltages. These then feed two resettable peak detectors which when one exceeds a threshold tells a PIC micro to read the two peak values, then reset the detectors for the next pass of the system.

NOW, the bit that's driving me up the wall is resolving the two I and Q (X and Y) signals into a phase angle and a phasor amplitude (rectangular to polar conversion). I cannot find an ATAN (Arctan) or Tan^1 function ANYWHERE in the know universe. Can any of my fellow "Boffins" help please?

I need to convert in the range -90 degrees to +90 degrees. Basically its a simple method of measuring signal lag/lead. If you know of a simpler way of doing this then I'd like to know please.

BTW, direct sampling of the signal is OUT as the PIC has to do other things as well, besides which I'm using a carrier of 14.88KHz.
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