ISO2-CMOS MT8880C/MT8880C-1
Increasing t
improves talk-off performance since
REC
FLOW
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
941
FHIGH
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
1633
DIGIT
1
D3
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
D2
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
D1
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
D0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
it reduces the probability that tones simulated by
speech will maintain a valid signal condition long
enough to be registered. Alternatively, a relatively
short t
with a long t
would be appropriate for
REC
DO
2
extremely noisy environments where fast acquisition
time and immunity to tone drop-outs are required.
Design information for guard time adjustment is
shown in Figure 6. The receiver timing is shown in
Figure 9 with a description of the events in Figure 11.
3
4
5
6
Call Progress Filter
7
A call progress mode, using the MT8880C/C-1, can
be selected allowing the detection of various tones
which identify the progress of a telephone call on the
network. The call progress tone input and DTMF
input are common, however, call progress tones can
only be detected when CP mode has been selected.
DTMF signals cannot be detected if CP mode has
been selected (see Table 5). Figure 8 indicates the
useful detect bandwidth of the call progress filter.
Frequencies presented to the input, which are within
the ‘accept’ bandwidth limits of the filter, are hard-
limited by a high gain comparator with the IRQ/CP
pin serving as the output. The squarewave output
obtained from the schmitt trigger can be analyzed by
8
9
0
*
#
A
B
C
D
a
microprocessor or counter arrangement to
determine the nature of the call progress tone being
detected. Frequencies which are in the ‘reject’ area
will not be detected and consequently the IRQ/CP
pin will remain low.
0= LOGIC LOW, 1= LOGIC HIGH
Figure 7 - Functional Encode/Decode Table
LEVEL
(dBm)
DTMF Generator
The DTMF transmitter employed in the MT8880C/C-
1 is capable of generating all sixteen standard DTMF
tone pairs with low distortion and high accuracy. All
frequencies are derived from an external 3.579545
MHz crystal. The sinusoidal waveforms for the
individual tones are digitally synthesized using row
and column programmable dividers and switched
capacitor D/A converters. The row and column tones
are mixed and filtered providing a DTMF signal with
low total harmonic distortion and high accuracy. To
specify a DTMF signal, data conforming to the
encoding format shown in Figure 7 must be written to
the transmit Data Register. Note that this is the same
as the receiver output code. The individual tones
-25
0
250
500
750
FREQUENCY (Hz)
= Reject
= May Accept
= Accept
Figure 8 - Call Progress Response
The period of each tone consists of 32 equal time
segments. The period of a tone is controlled by
varying the length of these time segments. During
write operations to the Transmit Data Register the 4
bit data on the bus is latched and converted to 2 of 8
coding for use by the programmable divider circuitry.
This code is used to specify a time segment length
which will ultimately determine the frequency of the
tone. When the divider reaches the appropriate
count, as determined by the input code, a reset pulse
is issued and the counter starts again. The number
which are generated (f
and f
) are referred to
LOW
HIGH
as Low Group and High Group tones. As seen from
the table, the low group frequencies are 697, 770,
852 and 941 Hz. The high group frequencies are
1209, 1336, 1477 and 1633 Hz. Typically, the high
group to low group amplitude ratio (pre-emphasis) is
2dB to compensate for high group attenuation on
long loops.
4-37
MITEL [ MITEL NETWORKS CORPORATION ]
