Download Silicon Laboratories Si2415 Specifications

AN93
U76 (GEN1)
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
trigger an off-hook intrusion detection (DCL). All bits in
U76 are read/write. (See Table 61.)
OHSR[15:9] sets the off-hook loop current sample rate
for intrusion algorithms in 40 ms units. The default value
is 25 (1 sec). The minimum recommended value is 5
(200 ms). The sample rate can be adjusted to much
lower values; however, the likelihood of false intrusion
detections increases sharply with sample rates less
than 520 ms.
Bit 8 (FACL). If FACL = 0b (default), the ACL register is
automatically updated to the LVCS value at the sample
rate 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 offhook conditions. If FACL = 1b, 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 offhook 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 offhook 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 = 0b, 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 0b.
U76 resets to 0x3240 with a power-on or manual reset.
(See Table 61.)
Table 61. U76 Bit Map
Bit
Name
Function
15:9
OHSR
Off-Hook Sample Rate for Intrusion Detection (40 ms units).
(1 second default)
8
FACL
Force ACL.
0 = While off-hook, ACL is automatically updated with LVCS value.
1 = While off-hook, ACL saves previously written value.
7:5
DCL
Differential Current Level (3 mA units). (6 mA default)
4:0
ACL
Absolute Current Level (3 mA units). (0 default)
86
Rev. 0.8