Maxim Integrated 73M1822/73M1922 Implementers Guide - Manual (Page 19)

UG_1x22_052 73M1822/73M1922 Implementer’s Guide
Rev. 1.0 19
4.4 Line-in-use and Loss of Battery Feed
When the FXO line is in the on-hook mode, the driver monitors the voltage on the line (typically 48 V in
the US) at regular intervals. A loss of battery voltage or a line-in-use event can affect the value of the line
voltage.
The loss of battery voltage results from disconnecting the phone line or a central office failure. In this
case, the line voltage falls to zero volts after a time constant.
The line-in-use event occurs when another phone connected to that line is going off-hook. The line
voltage drops to about 6 V when the line is in use.
The LV register (0x1B) can be polled to determine the line voltage. In an on hook case (and using the
reference schematics) the line voltage can be calculated as follows:
Line Voltage = ((LV[7:0] & 0xFE) * 1.13) + 1.4V
10M
100K
TIP
RNG
73M1x22B
DCI
412K
OFH
Figure 4: Line Voltage Measurement Circuit
The register which is read to determine the Line voltage is:
0x1B LV7 LV6 LV5 LV4 LV3 LV2 LV1 Res
Read Line Voltage[7:1]
1
2
3
4
5
6
7
8
9
BL_PAR
LV_MX
LU_PAR
Battery Feed
Battery Loss
Line-In-Use
Line Not In-Use
BF_PAR
Line
Voltage
time
Figure 3 : Battery Feed and Line-in-Use Detection