Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 162)

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162 Rev. 1.3
6.6.1.2. Line Not Present/In Use Indication (Method 2—Adaptive)
This method is enabled through %V2. This feature checks the line status before going off-hook and again before
dialing. While on-hook, the part monitors line voltage and updates U85 (5 (NLIU) [15:0] with this value.
Before going off-hook with the ATD, ATO, or ATA command, the ISOmodem reads the line voltage and compares it
with the stored reference.
To prevent polarity reversals from being detected as a loss of loop current, a debounce timer controlled by U-
registers 50 and 51 is used. However, if the HOI bit is set, a parallel phone intrusion while off-hook will give a LINE
IN USE result code to indicate that the ISOmodem has gone on hook due to a parallel phone intrusion.
6.6.2. Off-Hook Condition
When the ISOmodem is off-hook, the U79 [4:0] (LVCS) value represents loop current. Additionally, the ISOmodem
is typically in the data mode, and it is difficult for the host to monitor the LVCS value. For this reason, a controller-
based off-hook intrusion algorithm is used.
There is a delay between the ISOmodem going off-hook and the start of the intrusion algorithm set by
U77 [15:12] (IST) (Intrusion Settling Time). This avoids false intrusion detections due to loop transients during the
on-hook to off-hook transition. The off-hook intrusion algorithm monitors the value of LVCS at a sample rate
determined by U76 [15:9] (OHSR). The algorithm compares each LVCS sample to the reference value in
U76 [4:0] (ACL). ACL = 0 at the first off-hook event after reset unless a value is written to it by the host. If ACL = 0,
the ISOmodem does not begin the intrusion algorithm until after two LVCS samples have been received. If the host
writes a non-zero value to ACL prior to the ISOmodem going off-hook, a parallel phone intrusion occurring during
the IST interval and maintained until the end of the IST interval triggers a PPD interrupt. The ISOmodem also
automatically updates ACL with the LVCS value while off-hook if an intrusion has not occurred. An ACL value can
be written by the host and forced to remain unchanged by setting U76 [8] (FACL) = 1. If LVCS is lower than ACL by
an amount greater than the value set in U76 [7:5] (DCL) (6 mA default) for two consecutive samples,
U70 [2] (PPD), Parallel Phone Detect is set. If U70 [10] (PPDM) (Parallel Phone Detect Mask) is set to 1 (default
condition), the INT pin in UART mode or the INT bit (Hardware Interface Register 1, bit 3) in parallel or SPI mode is
also triggered. The host can monitor PPD or issue an AT:I to verify the cause of an interrupt and clear PPD. The
host can take the appropriate action when the intrusion is confirmed.
The intrusion detection algorithm is as follows:
if LVCS(t) = LVCS (t – 40 ms x OHSR)
and ACL – LVCS(t) < DCL
then ACL = LVCS(t)
if (ACL – LVCS x (t – 40 ms x OHSR) > DCL
and ACL – LVCS x t > DCL
then PPD = 1
and the INT pin (or the INT bit in parallel or SPI mode) is asserted (PPDM = 1)
The ISOmodem can also be programmed to go on-hook automatically on a PPD interrupt by setting
U77 (HOI) [11] (Hang-Up On Intrusion) to 1.
The off-hook intrusion algorithm may be suspended for a period defined by U78 [15:14] (IB) after the start of
dialing. This guards against false PPD detects due to dial pulses or other transients caused by Central Office
switching.
Table 102 lists the U registers and bits used for Intrusion Detection.
Loop Voltage Action
0 < LVCS < 6.25 % x U85 Report NO LINE and remain on-hook
6.25 % x U85 < LVCS < 85% x U85 Report LINE IN USE and remain on-hook
85% x U85 < LVCS Go off-hook and establish connection