Silicon Laboratories SI2493/57/34/15/04, SI2494/39 manual U71 IO1 Bit D15 D14 D13 D12 D11 D10 Name

AN93

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

Reset settings = 0x0000

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.

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).

114 Rev. 1.3