Data Sheet
The microphone input is differential with an input
impedance of 20 kΩ.
The soft clip circuit limits the output voltage at LI to 2V
PEAK
(see figure 12). The attack time is 30
µs/6
dB and the
decay time is 20 ms/6 dB. When mute is active, during
dialling or after pressing the
P/M
key, the gain is
reduced by > 60 dB.
Receive
The gain of the receive path is 3 dB for 600
Ω
line
termination (test circuit figure 10). The receive input is
the differential signal of RI and STB. When mute is active
during dialling the gain is reduced by > 60 dB. During
DTMF dialling a MF comfort tone is applied to the
receiver. The comfort tone is the DTMF signal with a
level that is -30 dB relative to the line signal.
The receive gain can be changed by pressing the
volume keys. The
VOL
key gives a +5.4 dB boost and
has a toggle function, i.e. repressing the key resets the
gain to default. As an alternative the
+/–
keys can be
used. The
+
key increases the gain by 8.1 dB in 3 steps
and the
–
key decreases the gain by 5.4 dB in 2 steps
(total range 13.5 dB). On AS2533/5 the volume is reset
to default by next off-hook and on AS2534/6 the volume
setting will remain at last setting. A POR will always reset
the volume to default.
The soft clip circuit limits the voltage at the receive
output (RO) to 1V
PEAK
(see figure 11). It prevents harsh
distortion and acoustic shock.
Sidetone
A good sidetone cancellation is achieved by using the
following equation:
AS2533...6
Line Loss Compensation
The line loss compensation is a pin option. When it is
activated, the transmit and receive gains are decreased
by 6 dB at line currents from 20 to 50 mA when LLC = A
GND
and 45 to 75 mA when LLC = high (@ R1 = 30
Ω).
The
line loss compensation is disabled when LLC = low (see
figure 4 and 5).
Dialling Functions
Valid Keys
The key scanning is enabled when HS/DPN is pulled
high and V
DD
is above V
REF
. A valid key is detected from the
keyboard by connecting the appropriate row to the
column (R
ON
< 1 kΩ). This can be done using an n x m
keyboard matrix with single contacts. Four diodes are
used to extend the number of rows (see keyboard
arrangement fig. 2). It is also possible to connect a
µcontroller to the rows and columns (see application
note AN3010).
P/M and MT Key
The
P/M
key (not AS2534E) is used to enter the
Programme/Mute state and the
MT
key for muting
(AS2534E only). Depressing the
P/M
key mutes the
speech circuit and the device is in programme state.
Repressing the
P/M
key deactivates the mute and
programme state (toggle function).The
MT
key works in
a similar way, but has no programme function.
When programme/mute is activated, the LED indication
is turned on (flashing: 80 ms 'on' and 1 sec 'off').
Dial Mode Selection
The default mode (LD or MF) can be selected by the
mode pin. When default LD mode is selected, a temporary
change to MF can be invoked by pressing
T/P
key (not
AS2535) or the
*
key (when mode pin = row 5, the MF
tone is generated with the first
*
key entry). The circuit
will revert to LD by repression the
T/P
key (not AS2535)
or by pressing the
R
(R1,
R2
or
R3)
key or by next on-
hook.
When MF mode is selected by the mode pin, the circuit
can not be changed temporary to LD but will remain in
MF mode.
Centrex Keys (not AS2535)
The alphameric keys accommodate easy use of centrex
services. The A, B, C and D keys are only valid in MF
mode (including temporary MF) and are not storable.
Pressing one of these keys will invoke the appropriate
MF tones to be transmitted.
Z
BAL
Z
2
=
Z
LINE
Z
1
The sidetone cancellation signal is applied to the STB
input.
Z
LINE
Z1
Z
BAL
Figure 3
Side Tone Balance
Z2
Rev. 7
6
July 1998
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
