SI2414 SILABS [Silicon Laboratories], SI2414 Datasheet - Page 36

• Manufacturer Part Number: SI2414
• Manufacturer: SILABS [Silicon Laboratories]

Si2456/Si2433/Si2414
enabled, the INT pin is activated. After 40 1s (marks)
have been detected, the Si2456/33/14 searches for a
start bit, echo “CIDM”, and begins assembling
characters and transmitting them out through the serial
port. When the carrier is lost, the Si2456/33/14
immediately hangs up and echoes “No Carrier”.
Force Caller ID Monitor (Always On)
The Si2456/33/14 may be used to continuously monitor
the phone line for the caller ID mark (1) FSK data. This
can be useful in systems that require detection of caller
ID data before the ring signal, voice mail indicator
signals, and Type II caller ID support. The +VCDT AT
command can be used to force the Si2456/33/14 into
this mode.
Parallel Phone Detection
The ISOmodem is able to detect when another
telephone, modem, or other device is using the phone
line. This allows the host to avoid interrupting another
phone call when the phone line is already in use and to
intelligently handle an interruption when the ISOmodem
is using the phone line.
On-Hook Intrusion Detection
When the ISOmodem is sharing the telephone line with
other devices, it is important that it not interrupt a call in
progress. To detect whether another device is using the
shared telephone line, the host can use the ISOmodem
to monitor the TIP-RING dc voltage with the LVCS (Line
Voltage and Current Sense) register (U79, bits 4:0). See
Figure 7 on page 37 .
Set U69[2] (MODE) = 1 to read LVCS while on-hook.
Before
U69[2] (MODE) = 0. The commands to accomplish this
are listed in Table 18.
36
AT:U69,0004<CR> Sets MODE bit to monitor loop
AT:U69,0000<CR> Clears MODE bit for modem to
AT Command
AT:R79<CR>
going
Table 18. AT Commands
off-hook,
Function
voltage.
U79[4:0] (LVCS) measures loop
voltage.
go off-hook.
it
is
necessary
to
set
Rev. 0.9
Off-Hook Intrusion Detection
When the ISOmodem is off-hook, an algorithm is
implemented in the ISOmodem to automatically monitor
the TIP-RING loop current via the LVCS register.
Because the TIP-RING voltage drops significantly when
off-hook, TIP-RING current is a better indicator of
intrusion than TIP-RING voltage. When the ISOmodem
is
representing the TIP-RING voltage to representing the
TIP-RING current. (See Figure 8 on page 38 .) Upon
detecting an intrusion, the ISOmodem alerts the host of
the condition via the INT pin (the INT bit in parallel
mode). The host must unmask this interrupt by setting
the PPDM bit (U70, bit 10). After detecting the interrupt,
the host should then issue the AT:I command to verify
the reason for the interrupt was a parallel phone
intrusion and to clear the PPD bit (U70, bit 2). The
ISOmodem may be set to automatically hang up on a
PPD interrupt by setting the HOI bit (U77, bit 11).
The off-hook intrusion algorithm monitors the value of
LVCS (U79, bits 4:0) at a sample rate determined by the
OHSR (U76, bits 15:9) register (40 ms units). The
algorithm compares each LVCS sample to the reference
value in the ACL register (U76, bits 4:0). If LVCS is
lower than ACL by an amount greater than DCL (U76,
bits 7:5), the algorithm waits for another LVCS sample,
and if the next LVCS sample is also lower than ACL by
an amount greater than DCL, a PPD interrupt occurs.
This helps the ISOmodem avoid a false PPD interrupt
due to glitches on the phone line. The ACL is continually
updated with the value of LVCS as outlined below. If
desired, the host may force ACL to a fixed value by
setting the FACL bit (U76, bit 8). The algorithm can be
outlined as follows:
If
then ACL = LVCS(t)
If
then PPD = 1 and INT (INT bit in parallel mode) is
asserted.
The very first sample of LVCS the algorithm uses after
going off-hook does not have any previous samples for
comparison. If LVCS was measured during a previous
call, this value of LVCS may be used as an initial
reference. ACL may be written by the host with this
known value of LVCS. If ACL is non-zero, the
ISOmodem uses ACL as the first valid LVCS sample in
the off-hook intrusion algorithm. If ACL is 0 (default after
LVCS(t) = LVCS(t – 40 ms x OHSR)
LCVS(t) – ACL > DCL
(ACL – LVCS[t – 40 ms x OHSR]) > DCL)
off-hook,
and
(ACL – LVCS[t]) > DCL)
and
the
LVCS
register
switches
from