>Hmm.  How tiny can you make a SW touchtone decoder if you only want to
    >decode four "buttons" (ie one row or one column of the matrix...)

     Why do you think I want to  decode only four?

I can think of any number of applications for fewer than all 12 (or 16)
separate functions triggerable over a phone line (ie, how many functions
does a typical answering machine really have?)  Traditionally, the approach
is to use an external decoder ("expensive" and does all 16 codes), or to do
a relatively complex SW algorithm (which we just heard can use a substantial
fraction of the available cycles on a PIC.)  A SW function to decode only
four buttons seems like a tradeoff of the sort that might be useful.

BillW