MT8888C
t
GTP
= (R
P
C1) In [V
DD
/ (V
DD
-V
TSt
)]
t
GTA
= (R1C1) In (V
DD
/V
TSt
)
V
DD
C1
St/GT
R
P
= (R1R2) / (R1 + R2)
Data Sheet
R1
ESt
R2
a) decreasing tGTP; (tGTP < tGTA)
t
GTP
= (R1C1) In [V
DD
/ (V
DD
-V
TSt
)]
V
DD
C1
St/GT
t
GTA
= (R
p
C1) In (V
DD
/V
TSt
)
R
P
= (R1R2) / (R1 + R2)
R1
ESt
R2
b) decreasing tGTA; (tGTP > tGTA)
Figure 6 - Guard Time Adjustment
The value of t
DP
is a device parameter (see AC Electrical Characteristics) and t
REC
is the minimum signal duration
to be recognized by the receiver. A value for C1 of 0.1
µF
is recommended for most 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.
6.0
Call Progress Filter
A call progress mode, using the MT8888C, 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 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.
7
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
