mov r0,_iap_freq

...maybe should it be
mov r0,#_iap_freq

It's almost unbelievable, I did such a silly mistake

No, it is very believable. I think this mistake is the one that, even seasoned, '51 programmers make most often.

A simple cure is to make the label of all constants CAPITALS and all variable labels something else. It does not keep you from making the mistake, but makes it a lot easier to spot.

Erik