MT8880C 데이터 시트보기 (PDF) - Zarlink Semiconductor Inc
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MT8880C Datasheet PDF : 22 Pages
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ISO2-CMOS MT8880C
Increasing tREC improves talk-off performance since
it reduces the probability that tones simulated by
speech will maintain a valid signal condition long
enough to be registered. Alternatively, a relatively
short tREC with a long tDO would be appropriate for
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.
Call Progress Filter
A call progress mode, using the MT8880C, 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
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.
DTMF Generator
The DTMF transmitter employed in the MT8880C 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
which are generated (fLOW and fHIGH) are referred to
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.
FLOW
FHIGH
DIGIT
D3
D2
D1
D0
697 1209
1
0001
697 1336
2
0010
697 1477
3
0011
770 1209
4
0100
770 1336
5
0101
770 1477
6
0110
852 1209
7
0111
852 1336
8
1000
852 1477
9
1001
941 1336
0
1010
941 1209
*
1011
941 1477
#
1100
697 1633
A
1101
770 1633
B
1110
852 1633
C
1111
941 1633
D
0000
0= LOGIC LOW, 1= LOGIC HIGH
Figure 7 - Functional Encode/Decode Table
LEVEL
(dBm)
-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
5
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