Specifications

AN93
148 Rev. 0.8
To set the Si2457 to monitor loop current in the off-hook
state, the host would issue the following commands:
Overcurrent Detection Example
The Si2493/57/34/15/04 has a built-in overcurrent
detection feature (disabled by default) that measures
loop current a programmable amount of time after going
off-hook. This allows the modem to detect an improper
line condition. The overcurrent detect feature is enabled
by setting U70[11] (OCDM) = 1
b
. During the time after
the modem goes off-hook, loop current is measured and
set by U77[8:0] (OHT). The default delay is 16 ms. After
the delay, current is sampled every 1 ms. An
overcurrent is detected if two consecutive samples
indicate an overcurrent condition. If this feature is
enabled and excessive current is detected, the Si2493/
57/34/15/04 sends the “X” result code and triggers an
interrupt by asserting the INT
pin or by setting the INT
bit in the parallel mode. After an interrupt is received,
the host issues the AT:I command to verify the OCD
interrupt and clear the OCD bit. The delay between
modem off-hook and loop current measurement is set
by the OHT bits. OHT is a 9-bit register with 1 ms units.
The default delay is 16 ms. When the modem is off-
hook in an overload condition, LVCS = 11111 (full
scale—overload error condition), an X is sent to the
DTE, and the OCD bit is set.
The Overcurrent Detection feature is controlled by
changing U-Register settings. The registers and bits
that control these features are shown in Table 98.
Pulse/Tone Dial Decision
There are three methods to detect whether a telephone
line supports DTMF dialing or pulse dialing only. The
first method, which is the simplest, may require the
modem to go off-hook more than once. The second
method is slightly more complicated but does not
require the modem to go off-hook multiple times.
Method #1: Multiple off-hook transitions:
Use DTMF to dial the desired number with the "ATDT"
command. If the line accepts tone dialing, the call is
completed, and connection to the remote modem
proceeds as usual.
If the line only allows pulse dialing, the modem hangs
up and reports "UN-OBTAINABLE NUMBER." This
indicates the modem detected a dial tone after the
DTMF dial attempt. Dial the number again using the
"ATDP" command instead of "ATDT" to use pulse
dialing.
Method #2: Single off-hook transition:
Use this method if it is undesirable for the modem to go
off-hook more than once or to DTMF dial a single digit.
This method is somewhat more complicated and is best
illustrated with an example, dialing the number 1234
below. This method only works with rev. F and later.
Set bit 7 of U-register 7A (U7A[7](DOP) = 1b) and send
ATDT1;<cr> (Dial the first digit using DTMF and wait for
a response). A response of “OK” indicates the DTMF
digit, 1, was sent, and you can continue. If a response of
“NO DIALTONE” is received, the command failed
because there was no dial tone (no line available), and
the call cannot be completed.
If a response of “OK” is received after sending
ATDT1;<cr>, continue by sending ATDTW;<cr> to
perform the 2nd dial tone detection and wait for a
response. A response of “NO DIALTONE” “OK”
indicates that no dial tone was detected for 2 seconds,
and the line is DTMF capable. Complete the dialing by
sending ATDT2345<cr> (DTMF dial beginning with the
second number since the first number was successfully
Command Function
AT:R79<CR> Host reads the loop voltage
from the LVCS Register U79
bits 4:0 while the modem is
on-hook.
Command Function
ATH1 To go off-hook
AT:R79 Host reads loop current from
the LVCS Register U79 bits
4:0 while the modem is off-
hook.
Table 98. Overcurrent Detection
Register Bit Value Function
U67 7 DCR DC Impedance Select
U70 11 OCDM Overcurrent Detect
Mask
U70 3 OCD Overcurrent Detect
U77 8:0 OHT Off-Hook Time
U79 4:0 LVCS Line Voltage Current
Sense