MT8870 Mitel Networks Corporation, MT8870 Datasheet - Page 4

• Manufacturer Part Number: MT8870
• Description: ISO2-CMOS Integrated DTMF Receiver
• Manufacturer: Mitel Networks Corporation

MT8870D/MT8870D-1
condition is maintained (ESt remains high) for the
validation period (t
(V
latching its corresponding 4-bit code (see Table 1)
into the output latch. At this point the GT output is
activated and drives v
high as long as ESt remains high. Finally, after a
short delay to allow the output latch to settle, the
delayed steering output flag (StD) goes high,
signalling that a received tone pair has been
registered. The contents of the output latch are made
available on the 4-bit output bus by raising the three
state control input (TOE) to a logic high. The
steering
interdigit pause between signals. Thus, as well as
rejecting signals too short to be considered valid, the
receiver will tolerate signal interruptions (dropout)
too short to be considered a valid pause. This facility,
together with the capability of selecting the steering
time constants externally, allows the designer to
tailor performance to meet a wide variety of system
requirements.
Guard Time Adjustment
In many situations not requiring selection of tone
duration and interdigital pause, the simple steering
circuit shown in Figure 4 is applicable. Component
values are chosen according to the formula:
The value of t
11) and t
recognized by the receiver. A value for C of 0.1 µF is
4-14
TSt
V
St/GT
ESt
V
St/GT
ESt
) of the steering logic to register the tone pair,
DD
DD
REC
Figure 5 - Guard Time Adjustment
circuit works in reverse to validate the
R
1
R
1
is the minimum signal duration to be
DP
C
C
1
R
1
is a device parameter (see Figure
R
2
t
2
GTP
REC
t
ID
t
c
GTP
=t
t
to V
GTP
=t
), v
a) decreasing t
b) decreasing t
DA
=(R
t
DP
GTA
t
=(R
GTA
+t
c
DD
+t
P
R
=(R
C
GTA
1
=(R
R
reaches the threshold
P
C
. GT continues to drive
GTP
1
=(R
P
)In[V
1
1
=(R
)In[V
P
C
C
1
1
)In(V
R
1
1
DD
)In(V
GTP
R
2
DD
GTA
)/(R
2
/(V
)/(R
/(V
DD
; (t
; (t
DD
DD
1
DD
+R
/V
GTP
1
GTP
/V
-V
+R
TSt
-V
ISO
2
TSt
TSt
)
<t
2
>t
TSt
)
)
GTA
)
)]
GTA
)]
2
)
)
-CMOS
L=LOGIC LOW, H=LOGIC HIGH, Z=HIGH IMPEDANCE
X = DON‘T CARE
recommended for most applications, leaving R to be
selected by the designer.
Different steering arrangements may be used to
select independently the guard times for tone
present (t
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
improves talk-off performance since it reduces the
probability that tones simulated by speech will
maintain signal condition long enough to
registered. Alternatively, a relatively short t
a long t
environments where fast acquisition time and
immunity to tone drop-outs are required. Design
information for guard time adjustment is shown in
Figure 5.
Digit
ANY
A
B
C
D
A
B
C
D
1
2
3
4
5
6
7
8
9
0
#
*
DO
TOE
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
L
GTP
Table 1. Functional Decode Table
would be appropriate for extremely noisy
) and tone absent (t
INH
H
H
H
H
X
X
X
X
X
X
X
X
X
X
X
X
X
L
L
L
L
ESt
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
L
L
L
L
undetected, the output code
will remain the same as the
previous detected code
Q
Z
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
4
GTA
Q
Z
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
3
). This may be
Q
Z
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
2
REC
Q
Z
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
with
REC
1
be