Si2401
Preliminary Rev. 0.9
17
sustains through the end of the interval triggers an
interrupt.7
The off-hook intrusion algorithm may additionally be
disabled for a period of time after dialing begins via the
IB register (S82, bits 2:1). This avoids triggering an
interrupt due to pulse dialing, open-switch intervals, or
line transients from central office switching. Intrusion
may be disabled from the start of dialing to the end of
dialing (IB = Dl
b
), from the start of dialing to the timeout
of the IS (S29, bits 7:0) by setting IB = 10
b
(IB = 2) or
from the start of dial to carrier detect by setting IB = 11.
The off-hook intrusion algorithm is only suspended (not
disabled) during this IB interval. Therefore, any intrusion
that occurs during the IB interval and sustains through
the end of the interval triggers a PPD interrupt.
Interrupt Detection
The INT interrupt pin can be programmed to alert the
host of loss-of-carrier, loss-of-phone-line voltage/
current, parallel phone detection, and other interrupts
listed in the interrupt status mask (S08). After the host
receives an interrupt via the INT pin, the host should
issue the AT:I command. This command causes a read-
clear of the WOR, PPD, NLD, RI, OCD, and REV bits of
the S09 register and raises (deactivates) the INT pin. All
the interrupt status bits in register S09 remain high after
being set until cleared by the AT:I command.
Loop Current Detection
In addition to monitoring parallel phone intrusion, it is
possible to monitor the loss of loop current. This feature
can be enabled by setting S08[4] (NLDM) = 1. This
feature is disabled by default. If the loop current is too
low for normal DAA operation, S09[4] (NLD) is set.
During this event, if the NLR result code is enabled by
setting S62[1](NLR) = 1, the “l” result code is sent. Once
the loop current returns to a normal current state, the “L”
result code is sent. The INT pin is also asserted if
enabled.
Loss-of-Carrier Detection
The Si2401 has two methods of implementing a loss-of-
carrier function. If GPIO4 is programmed as INT and if
S08[7](CDM)=1, INT asserts in data mode when a loss-
of-carrier is detected. The carrier detect function may
also be implemented on GPIO2 by setting SE2[3:2]
(GPIO2) = 01 and SOC[7](CDE) = 1.
Overcurrent Detection
The Si2401 has an integrated overcurrent detection
feature. The Si2401 begins monitoring for an
overcurrent condition at a programmable time set by
S32 (OCDT) after going off-hook (default = 20 ms). If an
overcurrent condition is detected, the Si2401 sets
S09[1] interrupt status. As long as GPIO4 is
programmed as INT and the overcurrent mask bit is
enabled by setting S08[1](OCDM) = 1, INT asserts
during an overcurrent situation. The host may then
check S09[1] (OCD) via the AT:I command to confirm
that an overcurrent condition occurred.
Caller ID Decoding Operation
The Si2401 supports full caller ID detection and decode
for US Bellcore and UK standards. To use the caller ID
decoding feature, the following configuration is
necessary:
1. Set SE0[3] (ND) = 0
b
(set modem to 8N1 configuration).
2. Set S0C[6:5] (CIDM) = 01 (set modem to Bellcore type
caller ID) or S13[2] (CIDB) = 1 (set modem to UK type
caller ID).
Caller ID Monitor/Bellcore Caller ID
The Si2401 continuously monitors the phone line for the
caller ID mark signals. 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
monitor support. To force the Si2401 into caller ID
monitor mode, set SOC[6:5] (CIDM) = 11.
Note:
CIDM should be disabled before going off-hook.
UK Caller ID Operation
The Si2401 starts searching for the Idle State Tone Alert
Signal. When this signal has been detected, the Si2401
transmits an “a” to the host. After the Idle State Tone
Alert Signal is completed, the Si2401 applies the
wetting pulse for the required 15 ms by quickly going
off-hook and on-hook. From this point on, the algorithm
is identical to that of Bellcore in that it searches for the
channel seizure signal and the marks before echoing an
“m” and then reports the decoded caller ID data.
V.23 Operation/V.23 Reversing
The Si2401 supports full V.23 operation including the
V.23 reversing procedure. V.23 operation is enabled by
setting S07 (MF1) = xx10xx00
b
or xx01xx00
b
. If
S07[5] (V23R) = 1, the Si2401 transmits data at 75 bps
and receives data at 600 or 1200 bps. If
S07[4] (V23T) = 1, the Si2401 receives data at 75 bps
and transmits data at 600 or 1200 bps. S07[2] (BAUD)
is the 1200 or 600 bps indicator. BAUD = 1 enables the
1200/600 V.23 channel to run at 1200 bps, while
BAUD = 0
b
enables 600 bps operation.
When a V.23 connection is successfully established, the
modem responds with a “c” character if the connection
is made with the modem transmitting at 1200/600 bps
and receiving at 75 bps. The modem responds with a
“v” character if a V.23 connection is established with the
modem transmitting at 75 bps and receiving at 1200/
600 bps.
The Si2401 supports the V.23 turnaround procedure.
