MT8889 Zarlink Semiconductor, Inc., MT8889 Datasheet - Page 5

• Manufacturer Part Number: MT8889
• Description: Integrated DTMF Transceiver with Adaptive Micro Interface
• Manufacturer: Zarlink Semiconductor, Inc.

applications, leaving R1 to be selected by the
designer. Different steering arrangements may be
used to select independent tone present (t
tone absent (t
necessary to meet system specifications which place
both accept and reject limits on tone duration and
interdigital pause. Guard time adjustment also allows
the designer to tailor system parameters such as talk
off and noise immunity.
Increasing t
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
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 7 with a description of the events in Figure 9.
Call Progress Filter
A call progress mode, using the MT8889C, can be
selected allowing the detection of various tones,
which identify the progress of a telephone call on the
network.
input are common, however, call progress tones can
only be detected when CP mode has been selected.
V
ESt
St/GT
RX
b3
b2
Read
Status
Register
IRQ/CP
in
EVENTS
0
-RX
REC
3
with a long t
The call progress tone input and DTMF
REC
improves talk-off performance since
GTA
t
REC
DECODED TONE # (n-1)
) guard times. This may be
DO
A
would be appropriate for
Figure 7 - Receiver Timing Diagram
t
DP
GTP
t
REC
) and
TONE #n
B
t
GTP
t
DTMF signals cannot be detected if CP mode has
been selected (see Table 7). 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
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.
PStRX
t
PStb3
LEVEL
(dBm)
microprocessor
t
# n
ID
-25
C
t
DA
t
Figure 8 - Call Progress Response
GTA
0
= Reject
= May Accept
= Accept
#n + 1
TONE
D
t
DO
or
250
FREQUENCY (Hz)
E
counter
TONE
#n + 1
# (n + 1)
500
MT8889C
arrangement
F
750
V
TSt
to
5