Maxim Integrated 73M1866B/73M1966B Implementers Guide - Manual (Page 22)

73M1866B/73M1966B Implementer’s Guide UG_1x66B_016
22 Rev. 1.3
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.
Figure 4: Battery Feed and Line-in-Use Detection
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
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