Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 114)

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114 Rev. 1.3
Reset settings = 0x0000
U76 provides control for parallel phone detect (PPD) intrusion parameters including the off-hook sample rate
(OHSR), absolute current level with modem off-hook (ACL), ACL update from LVCS (FACL), and the difference in
current between ACL and LVCS that triggers an off-hook intrusion detection (DCL). All bits in U76 are read/write
(see Table 74).
OHSR[15:9] sets the off-hook loop current sampling interval for intrusion algorithms in 40 ms units. The default
value is 25 (1 s). The minimum recommended value is 5 (200 ms). The interval can be adjusted to much lower
values; however, the likelihood of false intrusion detections increases sharply with intervals of less than 520 ms.
Bit 8 (FACL). If FACL = 0 (default), the ACL register is automatically updated to the LVCS value at the sampling
interval determined by OHSR. This feature is used to ensure the ACL value is continuously updated. Updating ACL
allows host software to determine the loop current (value returned in ACL) provided the modem is off-hook longer
than the time defined by U77 (IST). Loop current on a particular line can vary over time due to a variety of factors
including temperature and weather conditions. Updating ACL reduces the probability of false intrusion detection by
ensuring the ACL reference reflects the most recent off-hook conditions. If FACL = 1, a value can be written into
ACL by the host. This value is not updated and remains in the ACL register until overwritten by the host or until
FACL is returned to 0 and updates from LVCS overwrite the stored value. Writing an initial value to ACL eliminates
the possibility of the modem going off-hook for the first time simultaneously with an intrusion and storing the
intrusion loop current in ACL.
Bits 7:5 (DCL) set the differential level between ACL and LVCS that triggers an off-hook PPD interrupt. DCL is
adjustable in 3 mA units. The default value is 2 (6 mA).
Bits 4:0 (ACL): ACL provides a means of detecting a parallel phone intrusion during the time between the modem
going off-hook and the U77 [15:12] (IST) time value. If ACL = 0, the ISOmodem has no reference and must use the
loop current sample from the first off-hook event as a reference for parallel phone intrusion detection. Typically, the
host sets ACL to an approximate value and FACL = 0 before the first off-hook event after powerup or reset. This
allows the updated ACL value to be used for subsequent calls and eliminates a potential detection problem if an
intrusion occurs simultaneously when the modem goes off-hook for the first time after a powerup or reset. If
ACL = 0, it is ignored by the off-hook intrusion algorithm. A PPD interrupt is generated if U79 [4:0] (LVCS) is DCL
less than ACL for two consecutive samples. The ISOmodem writes ACL with the contents of LVCS after an
intrusion with the last LVCS value before the intrusion. The default value for ACL is 0.
U76 resets to 0x3240 with a power-on or manual reset (see Table 74).
U71 IO1
Bit D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Name COMP PRT
Type R/W R/W
Bit Name Function
15:5 Reserved Read returns zero.
4 COMP 0 – Disables compression (PCM mode).
1 – Enables linear compression.
3:1 Reserved
0 PRT 0 – Disables PCM mode.
1 – Enables PCM mode.