National Semiconductor NS TP5088 Tone Dialer chip provides
simple means DTMF generation and simple interfacing with uProcessor. Above
is a truth table for DTMF generation TABLE. The Tone Output frequency is
for TP5088 generation, not a standard, but it works with all most DTMF
decoder chip, say MT8870. Please note that the CODE for KEYBOARD equivalent
for number 0 is 10D or 1010B.
A typical application circuit
interfacing with microprocessors for direct generation of DTFM signal connected
to phone line through 600:600 Ohm transformer. The output tone level can
be adjusted by changing Re value..
