Specifications

AN93
92 Rev. 0.9
Reset settings = 0x0000
3.3.40. 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
triggers 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 = 0
b
(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 off-
hook conditions. If FACL = 1
b
, 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
3OCDOvercurrent Detect (sticky).
1 = Overcurrent condition has occurred. Clears on :I read.
2 PPD Parallel Phone Detect (sticky).
1 = Parallel phone detected since last off-hook event. Clears on :I read.
1RIRing Indicator (sticky).
1 = Ring event has occurred (Si2493/57/34/15/04 on-hook). Clears on :I read.
0DCDData Carrier Detect (status).
1 = carrier detected (inverse of DCD
pin).
Table 60. U70 Bit Map (Continued)
Bit Name Function
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.
4COMP0 – Disables compression (PCM mode).
1 – Enables linear compression.
3:1 Reserved
0PRT0 – Disables PCM mode.
1 – Enables PCM mode.