SA589 FAMILY

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Oscillator Circuit

This requires an external 560kHz ceramic resonator connected between OSC IN and OSC

OUT to provide a timing reference for all chip functions. No other components are required

or should be used.

The oscillator circuit is of the 'single pin' type. Internally, the OSC IN pin is connected to V

SS

and therefore the resonator may alternatively be connected externally between OSC OUT and

V

Please consult your resonator supplier who will recommend a suitable resonator type.

Tone Filtering

The spectral purity of the DTMF output is sufficient for most applications. However, where

lower distortion DTMF tones are required, an on-chip darlington pair is provided (accessible

via pins 7 & 8) for use in a low pass active filter.

Fig. 4 shows how a 2-pole Sallen and Key filter can be implemented. The typical component

values have been chosen to give a second order Butterworth response with a cut-off frequency

of about 3,5kHz and a nominal pass-band insertion loss of 0.5dB.

DTMF DIALLING

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 DC level

of 0.9 V

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 (unless

specified otherwise). 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 memory occurs at the

maximum rate.

If a key is held down for longer than the minimum Tone Burst period, the tone output will

continue until the key is released.