Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 116)

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116 Rev. 1.3
U79 is a bit-mapped register. Bits 15:6 are reserved.
Bits 5:0 represent the line voltage, loop current, or on-hook line monitor (see Table 77). While the modem is on-
hook, the value in the LVCS register measures loop voltage (see Table 78). This value can be used to determine if
a line is connected or if a parallel phone or other device goes off-hook or on-hook. The accuracy of the LVCS bits is
±20%. When the modem goes off-hook, the value in the LVCS register measures loop current. LVCS can indicate
when a parallel phone or other device goes on-hook or off-hook and detect whether enough loop current is
available for the modem to operate or if an overload condition exists.
The line voltage monitor full scale may be modified by changing R5 as follows:
V
MAX
= V
MIN
+ 4.2 x (10 M Ω + R5 + 1.78 kΩ) / (R5 + 1.78 kΩ) / 5
See Table 78. LVCS is backward-compatible with older ISOmodem revisions. The value is absolute and does not
reflect loop polarity. See U6C (LVS) [15:8] for 1 V/bit resolution and signed, two’s complement format and
U63 (LCS) [15:8] for 1.1 mA/bit loop current measurement. The values for loop voltage and loop current in U79 are
calculated by the modem from the values in U6C and U63 respectively.
U7A is a bit-mapped register. U7A resets to 0x0000. Bits 12, 10:8, and 5:3 are reserved.
Bit 7 (DOP) is used in a method to determine whether a phone line supports DTMF or only pulse dialing. See "6.10.
Pulse/Tone Dial Decision" on page 169 for details.
Table 76. U78 Bit Map
Bit Name Function
15:14 IB Intrusion Blocking.
00 = No intrusion blocking.
01 = Intrusion disabled from start of dial to end of dial.
10 = Intrusion disabled from start of dial to IS register time-out.
11 = Intrusion disabled from start of dial to CONNECT XXX, NO DIALTONE, or
NO CARRIER
13:8 Reserved Read returns zero.
7:0 IS Intrusion Suspend (500 ms units) default = 0 ms.
Table 77. Monitor Mode Values
On-Hook Voltage Monitor Mode Off-Hook Current Monitor Mode
00000 = No line connected.
00001 = Minimum line voltage (VMIN = 2.5 V ±0.5 V).
11111 = Maximum line voltage (87 V ±20%) 00000 = No loop current.
00001 = Minimum loop current.
11110 = Maximum loop current.
11111 = Overload (more than 60 mA in legacy TBR21
mode or more than 155 mA in other modes)
Table 78. U79 Bit Map
Bit Name Function
15:6 Reserved Read returns zero.
5:0 LVCS Line Voltage Current Sense.
On-Hook = Voltage Monitor (2.75 V/bit).
Off-Hook = Loop Current Monitor (3 mA/bit).