MT88L70 Data Sheet
5
Zarlink Semiconductor Inc.
Figure 3 - Basic Steering Circuit
Different steering arrangements may be used to select independently the guard times for tone present (t
GTP
) and
tone absent (t
GTA
). This may be necessary to meet system specifications which place both accept and reject limits
on both 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 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
4.
Power-down and Inhibit Mode
A logic high applied to pin 6 (PWDN) will power down the device to minimize the power consumption in a standby
mode. It stops the oscillator and the functions of the filters.
Inhibit mode is enabled by a logic high input to the pin 5 (INH). It inhibits the detection of tones representing
characters A, B, C, and D. The output code will remain the same as the previous detected code (see Table 1).
Figure 4 - Guard Time Adjustment
C
v
c
V
DD
St/GT
ESt
StD
MT88L70
R
t
GTA
=(RC)In(V
DD
/V
TSt
)
t
GTP
=(RC)In[V
DD
/(V
DD
-V
TSt
)]
V
DD
V
DD
St/GT
ESt
R
1
C
1
R
2
V
DD
St/GT
ESt
C
1
R
1
R
2
a) decreasing t
GTP
; (t
GTP
< t
GTA
)
b) decreasing t
GTA
; (t
GTP
> t
GTA
)
t
GTA
=(R
1
C
1
) In (V
DD
/ V
TSt
)
t
GTP
=(R
P
C
1
) In [V
DD
/ (V
DD
-V
TSt
)]
R
P
= (R
1
R
2
) / (R
1
+ R
2
)
t
GTA
=(R
P
C
1
) In (V
DD
/ V
TSt
)
t
GTP
=(R
1
C
1
) In [V
DD
/ (V
DD
-V
TSt
)]
R
P
= (R
1
R
2
) / (R
1
+ R
2
)
