TMS320C54CST datasheet(97/171 Pages) TI

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Functional Overview

November 2001 − Revised September 2003

SPRS187B

Table 3−46. Loop Current Transfer Function

LVCS[4:0]

CONDITION

00000

Insufficient line current for normal operation. Use the

DOD bit to determine if a line is still connected.

00001

Minimum line current for normal operation.

11111

Loop current is excessive (overload).

Overload > 140 mA in all modes except CTR21.

Overload > 54 mA in CTR21 mode.

3.19.2.10 Off-Hook

The communication system generates an off-hook command by setting the OH bit. With the OH bit set, the

system is in an off-hook state.

The off-hook state is used to seize the line for incoming/outgoing calls and can also be used for pulse dialing.

When the DAA is on-hook, negligible DC current flows through the hookswitch. When the DAA is placed in

the off-hook state, the hookswitch transistor pair, Q1 and Q2, turn on. This applies a termination impedance

across TIP and RING and causes DC loop current to flow. The termination impedance has both an AC and

DC component.

When executing an off-hook sequence, the DAA requires 1548/Fs seconds to complete the off-hook and

provide phone-line data on the serial link. This includes the 12/Fs filter group delay. If necessary, for the

shortest delay, a higher Fs may be established prior to executing the off-hook, such as an Fs of 10.286 kHz.

The delay allows for line transients to settle prior to normal use.

3.19.2.11 DC Termination

The DAA has four programmable DC termination modes which are selected with the DCT[1:0] bits in

FCC mode (DCT[1:0] = 10 b), shown in Figure 3−53, is the default DC termination mode and supports a

transmit full scale level of –1 dBm at TIP and RING. This mode meets FCC requirements in addition to the

requirements of many other countries.

FCC DCT Mode

.01 .02 .03 .04 .05 .06 .07 .08 .09

.11

Loop Current (A)

Figure 3−53. FCC Mode I/V Characteristics

CTR21 mode (DCT[1:0] = 11 b), shown in Figure 3−54, provides current limiting while maintaining a transmit

full scale level of –1 dBm at TIP and RING. In this mode, the DC termination will current limit before reaching

60 mA if the LIM bit is set.

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