SA541

sames

11/16

DTMF Tone Filtering

Figure 3: 2-Pole Sallen and Key Filter Circuit

9.

DTMF Dialling (Figure 4)

During DTMF dialling, the MUTE output goes to logic ‘1’ and remains there for the

duration of the tone transmission.

The IMP output remains low during tone

transmission. The MF OUT output rises to its d.c. level of 0,9 V

tone transmission and is kept there between tone bursts. This is to avoid transients

at the beginning and end of tone bursts.

The maximum rate at which tones are sent to line is 100ms on, followed by 100ms

off. If keys are activated faster than this, they are placed in a temporary store and

then sent to line at the maximum rate. Dialling from the LNR store occurs at the

maximum rate.

If a key is held down for longer than 100ms, the tone output will continue until the

key is released.