MT8889C
applications, leaving R1 to be selected by the
designer. Different steering arrangements may be
used to select independent tone present (t
GTP
) and
tone absent (t
GTA
) guard times. This may be
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
REC
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 t
REC
with a long t
DO
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 7 with a description of the events in Figure 9.
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 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.
LEVEL
(dBm)
-25
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. The call progress tone input and DTMF
input are common, however, call progress tones can
only be detected when CP mode has been selected.
EVENTS
t
REC
V
in
t
DP
ESt
t
GTP
A
t
REC
TONE #n
t
DA
B
t
ID
C
0
= Reject
250
500
750
FREQUENCY (Hz)
= May Accept
= Accept
Figure 8 - Call Progress Response
D
t
DO
TONE
#n + 1
E
F
TONE
#n + 1
t
GTA
V
TSt
t
PStRX
St/GT
RX
0
-RX
3
DECODED TONE # (n-1)
#n
t
PStb3
# (n + 1)
b3
b2
Read
Status
Register
IRQ/CP
Figure 7 - Receiver Timing Diagram
5
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
