Is There A Megafunction For Arctan ?

With 18bits a LUT is no feasible.

There are many options on the literature.

One is Cordic as previosuly reported.

Another option is a mix of ROM and linear (or quadratic interpolation).

Unfortunately it is not a simple task and a good approximation requires a careful analysis of the literature.

My group published many papers on these topics. As example:

1)

direct digital frequency synthesizers with polynomial hyperfolding technique (http://www.alteraforum.com/search/srchabstract.jsp?tp=&arnumber=1315881&querytext%3dde+caro%26refinements%3d4294967131%26openedrefinements%3d*%26searchfield%3dsearch+all)

De Caro, D.; Napoli, E.; Strollo, A.G.M.;

circuits and systems ii: express briefs, ieee transactions on (http://www.alteraforum.com/forum/../xpl/recentissue.jsp?punumber=8920)

Volume: 51 , issue: 7 (http://www.alteraforum.com/forum/../xpl/tocresult.jsp?isnumber=29164)

Digital Object Identifier: 10.1109/tcsii.2004.829553 (http://dx.doi.org/10.1109/tcsii.2004.829553)

Publication Year: 2004 , Page(s): 337 - 344

IEEE Journals

2)

high-performance special function unit for programmable 3-d graphics processors (http://www.alteraforum.com/search/srchabstract.jsp?tp=&arnumber=4694047&querytext%3dde+caro%26refinements%3d4294967131%26openedrefinements%3d*%26searchfield%3dsearch+all)

De Caro, D.; Petra, N.; Strollo, A.;

circuits and systems i: regular papers, ieee transactions on (http://www.alteraforum.com/forum/../xpl/recentissue.jsp?punumber=8919)

Volume: 56 , issue: 9 (http://www.alteraforum.com/forum/../xpl/tocresult.jsp?isnumber=5230335)

Digital Object Identifier: 10.1109/tcsi.2008.2010150 (http://dx.doi.org/10.1109/tcsi.2008.2010150)

Publication Year: 2009 , Page(s): 1968 - 1978