You can replace 8880 and 8870 with µC, both PIC and AVR can make DTMF using PWM, and of course to decode touch tones. Even ATTiny and PIC12F series of µC can do the job. 8880 is unduly expensive IC for that job, and 8870 is obsolete, maybe both.
Appreciate it, guys but i do know how to use the MT8880C.
It's just this one case which i'll be using only the dtmf generator function of the MT8880C. The control pins are permanently set to write mode. It seems that leaving those pins opened does not give any adverse effect on the chip itself and the dtmf generation seems to work ok.
I fear it is unwise to leave the 'unrelated pins' 'opened'. How shall i terminate them if otherwise.