20

4872A–CORD–08/05

U4091BM-R

12.5

Summary of Truth Table

1.

The circuit will switch to transmit mode if

– Both transmit level detectors sense higher signal levels than the respective receive

level detectors, and

– The transmit background-noise monitor indicates the presence of speech

2.

The circuit will switch to receive mode if

– Both receive level detectors sense higher signal levels than the respective transmit

level detectors, and

– The receive background-noise monitor indicates the presence of speech

3.

The circuit will switch to the reverse mode if

– The level detectors disagree on the relative strengths of the signal levels, and

– At least one of the background-noise monitors indicates speech

4.

The circuit will switch to idle mode when

– Both speakers are quiet (no speech present), or

– When one speaker speech level is continuously overridden by noise at the other

speaker’s location

The time required to switch the circuit between transmit, receive and idle is determined by inter-

nal current sources and the capacitor at pin CT. A diagram of the CT circuitry is shown in Figure

12-7 on page 21. It operates as follows:

• CCT is typically 4.7 µF.

• To switch to transmit mode, ITX is turned on (IRX is off), charging the external capacitor to

–240 mV below VM. (An internal clamp prevents further charging of the capacitor.)

• To switch to receive mode, IRX is turned on (ITX is off), increasing the voltage on the

capacitor to +240 mV with respect to VM.

• To switch to reverse mode, the current sources ITX, IRX are turned off, and the current

source IFI is switched on, discharging the capacitor to VM.

• To switch to idle mode, the current sources ITX, IRX, IFI are turned off, and the current

source ISI charges the capacitor to VM.