MT8885
Data Sheet
count, as determined by the input code, a reset pulse is issued and the counter starts again. The number of time
segments is fixed at 32, however, by varying the segment length as described above, the frequency can also be
varied. The divider output clocks another counter, which addresses the sinewave lookup ROM.
The lookup table contains codes which are used by the switched capacitor D/A converter to obtain discrete and
highly accurate DC voltage levels. Two identical circuits are employed to produce row and column tones, which
are then mixed by using a low noise summing amplifier. A bandwidth limiting filter is incorporated and serves to
attenuate distortion products above 8 kHz. Figure 10 shows that the distortion products are very low in amplitude.
Scaling Information
10 dB/Div
Start Frequency = 0 Hz
Stop Frequency = 3400 Hz
Marker Frequency = 697 Hz and
1209 Hz
Figure 10 - Spectrum Plot
4.0
Burst Mode
In certain telephony applications it is required that DTMF signals being generated are of a specific duration
determined either by the particular application or by any one of the exchange transmitter specifications currently
existing. Standard DTMF signal timing can be accomplished by making use of the Burst Mode. The transmitter is
capable of issuing symmetric bursts/pauses of predetermined duration. This burst/pause duration is 51 ms±1 ms
which is a standard interval for autodialer and central office applications. After the burst/pause has been issued, the
appropriate bit is set in the Status Register to indicate that the transmitter is ready for more data. The timing
described above is available when DTMF mode has been selected. However, when CP mode (Call Progress mode)
is selected, the burst/pause duration is doubled to 102 ms
±2
ms. Note that when CP mode and Burst mode have
been selected, DTMF tones may only be transmitted and
not
received. In applications where a non-standard
burst/pause time is desirable, a software timing loop or external timer can be used to provide the timing pulses
when the burst mode is disabled by enabling and disabling the transmitter.
5.0
Single Tone Generation
A single tone mode is available whereby individual tones from the low group or high group can be generated. This
mode can be used for DTMF test equipment applications, acknowledgment tone generation and distortion
measurements. Refer to Control Register B description for details.
10
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
