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On-hook Conditions
When a line is not in use (on-hook), the
designer may select either the speech
battery or the ringing battery as the sup-
ply to the line drivers. In this mode, most
of the internal circuits are put into a low
power operating mode, to minimise
supply currents. The A and B wire out-
puts are effectively connected to the
supply voltage, thus applying this volt-
age (minus a small saturation voltage)
to the line. The output is current limited
in this mode, thus protecting against
short circuits, and limiting any inrush
current when a set goes off hook. If the
SH LIC detects a current in excess of a
(programmable) limit, the off-hook con-
dition will be detected (an on-chip
debouncer with selectable delay avoids
accidental hookswitch detection), and
the circuit will be put into active speech
mode. The nominal off-hook detection
currents, and the hysteresis, are shown
in fig 7. When in the on-hook condi-
tion, the system designer may select,
Table 6: On-hook Characteristics
Parameter
Conditions
Vfeedo
Open-line feed voltage
Ion
Line current guaranteeing on-hook state
Ioff
Line current guaranteeing off-hook state
Iohyst
Hookswitch detect hysteresis
Thks
Hookswitch detect time
Ioc
Peak over-current limit, on-hook mode
VbiasH
Bias voltage during ADSI mode
VbiasL
on a (H) and b (L) wires ref. BAT pin
Notes:
1. These are the default values, after
reset. The on-hook and off-hook thresh-
olds can be individually programmed in
the range 0 to 63mA nominal.
2. Time between off-hook condition and
the line current reaching 90% of its final
value.
under program control via the GCI bus,
an 'on hook active mode', whereby on-
hook signalling (ADSI, CLI etc..) can be
performed in either direction (note, how-
ever, that battery reversal is not avail-
able in this mode.).
off-hook
on-hook
6.0
Fig. 7: Nominal hook-switch detection thresholds (default values)
Min
Vbats-1
5.1
8.2
2
2
3. This is the intrinsic current limit of the
output driver. This current can only be
seen during on-hook to off-hook tran-
sients, or during ringing into a short-cir-
cuit load during the ring-trip delay peri-
od. The actual value measured will
depend on the load resitance used.
4. Iline=0mA, independent of battery
reversal mode. This voltage is selected
15
MTK-40131
line current
mA
6.3
10.0
8.8
10.2
13.6
Max
Units
Vbatr
V
11.1
mA
14.2
mA
mA
20
ms
145
mA
4
V
by the user. The output impedance
when in the on-hook condition is set by
the sense resistors Rfeed.
The hook-switch detector has a pro-
grammable debounce timer. Times of 8,
16, 24 or 64 ms can be selected (com-
mon for both channels).
Note
4
1
1
1
2
3
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