SI3050 datasheet(29/128 Pages) SILABS

29 / 128 page

Si3050 + Si3011/18/19

Rev. 1.5

5.12.2. Loop Current Measurement

When the Si3050 is off-hook, the LCS[4:0] bits measure

loop current in 3.3 mA/bit resolution. With the LCS[4:0]

bits, a user can detect another phone going off-hook by

monitoring the dc loop current. The line current sense

transfer function is shown in Figure 21 and is detailed in

Table 14. The LCS and LCS2 bits report loop current

down to the minimum operating loop current for the

DAA. Below this threshold, the reported value of loop

current is unpredictable. The minimum operating loop

current of the DAA is set by the MINI[1:0] bits in

When the LCS bits reach max value, the Loop Current

Sense Overload Interrupt bit (Register 4) fires. LCSOI

firing however, does not necessarily imply that an

overcurrent situation has occurred. An overcurrent

situation in the DAA is determined by the status of the

OPD bit (Register 19). After the LCSOI interrupt fires,

the OPD bit should be checked to determine if an

overcurrent situation exists. The OPD bit indicates an

overcurrent situation when loop current exceeds either

160 mA (ILIM = 0) or 60 mA (ILIM = 1), depending on

the setting of the ILIM bit (Register 26).

The LCS2 register also reports loop current in the

off-hook state. This register has a resolution of 1.1 mA

per bit.

5.13. Off-Hook

The communication system generates an off-hook

command by setting the OH bit (Register 5, bit 0). This

off-hook state seizes the line for incoming/outgoing

calls. It also can be used for pulse dialing.

With the OH bit at logic 0, negligible dc current flows

through the hookswitch. When a logic 1 is written to the

OH bit, the hookswitch transistor pair, Q1 and Q2, turn

on. A termination impedance across TIP and RING is

applied and causes dc loop current to flow. The

termination impedance has both an ac and a dc

component.

Several events occur in the DAA when the OH bit is set.

There is a 250 µs latency for the off-hook command to

be communicated to the line-side device. When the

line-side device goes off-hook, an off-hook counter

forces a delay to allow line transients to settle before

transmission or reception can occur. The off-hook

counter time is controlled by the FOH[1:0] bits

(Register 31, bits 6:5). The default setting for the

off-hook counter time is 128 ms, but can be adjusted up

to 512 ms or down to 64 or 8 ms.

After the off-hook counter expires, a resistor calibration

is performed for 17 ms to allow the DAA internal

circuitry to adjust to the exact conditions present at the

time of going off-hook. This resistor calibration can be

disabled by setting the RCALD bit (Register 25, bit 5).

After the resistor calibration is performed, an ADC

calibration is performed for 256 ms. This calibration

helps to remove offset in the A/D sampling the

telephone line. ADC calibration can be disabled by

setting the CALD bit (Register 17, bit 5). See "5.6.

Calibration" on page 26 for more information on

automatic and manual calibration.

Silicon Laboratories recommends that the resistor and

the ADC calibrations not be disabled except when a fast

response is needed after going off-hook, such as when

responding to a Type II Caller-ID signal. See "5.25.

Caller ID" on page 36 for detailed information.

To calculate the total time required to go off-hook and

start transmission or reception, include the digital filter

delay (typically 1.5 ms with the FIR filter) in the

calculation.

5.14. Ground Start Support

The Si3050 DAA supports loop-start applications by

default. It can also support ground-start applications

with the RG, TGD, and TGDE pins and the schematic

shown in Figure 22. The component values are listed in

Table 15.

Table 14. Loop Current Transfer Function

LCS[4:0]

Condition

00000

Insufficient line current for normal operation.

Use the DOD bit (Register 19, bit 1) to

determine if a line is still connected.

00100

Minimum line current for normal operation.

(MINI[1:0] = 01)

11111

Loop current may be excessive. Use the

OPD bit to determine if an overload condi-

tion exists.

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