SI3232 datasheet(32/128 Pages) SILABS

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Si3232

Preliminary Rev. 0.96

4.5.1. Loop Closure Detection

Loop closure detection is required to accurately signal a

terminal device going off-hook during the Active or On-

Hook Transmission linefeed states (forward or reverse

polarity). The functional blocks required to implement a

loop closure detector are shown in Figure 14, and the

provided in Table 19. The primary input to the system is

the Loop Current Sense value provided by the voltage/

current/power monitoring circuitry and reported in the

ILOOP RAM address. The loop current (ILOOP) is

computed by the ISP using the equations shown below.

Refer to Figure 11 on page 26 for the discrete bipolar

transistor references used in the equation below (Q1,

Q2, Q5 and Q6 – note that the Si3200 has

corresponding MOS transistors). The same ILOOP

equation applies to the discrete bipolar linefeed as well

as the Si3200 linefeed device. The following equation is

conditioned by the CMH status bit in register LCRRTP

and by the linefeed state as indicated by the LFS field in

the LINEFEED register.

If the CMHITH (RAM 36) threshold is exceeded, the

CMH bit is 1, and IQ1 is forced to zero in the

FORWARD-ACTIVE and TIP-OPEN states, or IQ2 is

forced to zero in the REVERSE-ACTIVE and RING-

OPEN states. The other currents in the equation are

allowed to contribute normally to the ILOOP value.

The conditioning due to the CMH bit (LCRRTP Register)

and LFS field (LINEFEED Register) states can be

summarized as follows:

IQ1 = 0 if (CMH = 1 AND (LFS = 1 OR LFS = 3))

IQ2 = 0 if (CMH = 1 AND (LFS = 5 OR LFS = 7))

The output of the Input Signal Processor is the input to a

programmable digital low-pass filter, which can be used

to remove unwanted ac signal components before

threshold detection.

The low-pass filter coefficient is calculated using the

following equation and is entered into the LCRLPF RAM

location.

LCRLPF = [(2

πf x 4096)/800] x 23

Where f is the desired cutoff frequency of the filter.

The programmable range of the filter is from 0h (blocks

all signals) to 4000h (unfiltered). A typical value of 10

(0A10h) is sufficient to filter out any unwanted ac

artifacts while allowing the dc information to pass

through the filter.

The output of the low-pass filter is compared to a

programmable threshold, LCROFFHK. Hysteresis is

enabled

programming

second

threshold,

LCRONHK, to detect the loop going to an OPEN or on-

hook state. The threshold comparator output feeds a

programmable debounce filter. The output of the

debounce filter remains in its present state unless the

input remains in the opposite state for the entire period

of time programmed by the loop-closure debounce

interval, LCRDBI. There is also a loop-closure mask

interval, LCRMASK, that is used to mask transitions

caused when an internal ringing burst (no dc offset)

ends in the presence of a high REN load. If the

debounce interval has been satisfied, the LCR bit will be

set to indicate that a valid loop closure has occurred.

Table 18. 3-Battery Switching Components

Component

Value

Comments

200 V, 200 mA

IN4003 or similar

100 V PNP

CXT5401 or similar

100 V NPN

CXT5551 or similar

R101

1/10 W, ±5%

2.4 k

Ω for VDD =3.3 V

3.9 k

Ω for VDD =5 V

R102

10 k

Ω, 1/10 W, ±5%

R103

402 k

Ω, 1/10 W, ±1%

Iloop

IQ1 IQ6

–

IQ5 IQ2 in TIP-OPEN or RING-OPEN

IQ1 IQ6

–

IQ5 IQ2

–

---------------------------------------------------- in all other states

–

SI3232 Datasheet(PDF) 32 Page - Silicon Laboratories

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