How do I bind C-= in emacs?

This s-expression in my .emacs file does not produce the desired result:

(define-key global-map (kbd "C-=") 'djhaskin987-untab-to-tab-stop)

Why can't I bind a command to Ctrl+=?

EDIT for clarification:

I am using emacs23-nox on the standard build of urxvt-256colors for Debian, except that I have recompiled with --disable-iso405776 (or something to that effect) it so that Ctrl+Shift doesn't do the weird 'insert character' thing. I don't know if this affects anything. For example, C-M-i sends M-TAB, which I don't understand.

EDIT II:

I apologize for not making this clear. The function djhaskin987-untab-to-tab-stop has the line (interactive) in it. This part works.

Solution

In a terminal, TAB is represented by the same byte sequence as C-i. And typically the terminal has no special byte-sequence for C-=, so it will just send a =. There is nothing that Emacs can do about it. But you might be able to teach your terminal emulator to send some special byte sequence of your choice (check the documentation of your terminal emulator for that), after which you can teach Emacs to recognize it as a C-= (with something like (define-key input-decode-map "...thebytes..." [?\C-=])).