Advance Information
signal duration to be recognized by the receiver. A
value for C1 of 0.1 µF is recommended for most
MT88L85
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.
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)
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)
Call Progress Filter
R1
R2
b) decreasing tGTA; (tGTP > tGTA)
ESt
Figure 6 - Guard Time Adjustment
A call progress mode, using the MT88L85, 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.
C
D
E
F
EVENTS
A
B
t
REC
V
in
t
REC
TONE #n
t
ID
TONE
#n + 1
t
DO
TONE
#n + 1
t
DP
ESt
t
DA
t
GTP
t
GTA
V
TSt
St/GT
t
PStRX
RX
0
-RX
3
DECODED TONE # (n-1)
#n
# (n + 1)
t
PStb3
b3
b2
Read
Status
Register
IRQ/CP
Figure 7 - Receiver Timing Diagram
4-75
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