u4091bm-r ATMEL Corporation, u4091bm-r Datasheet - Page 20

• Manufacturer Part Number: u4091bm-r
• Description: Programmable Telephone Audio Processor
• Manufacturer: ATMEL Corporation

12.5
20
Summary of Truth Table
U4091BM-R
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
12-7 on page
1. The circuit will switch to transmit mode if
2. The circuit will switch to receive mode if
3. The circuit will switch to the reverse mode if
4. The circuit will switch to idle mode when
• CCT is typically 4.7 µF.
• To switch to transmit mode, ITX is turned on (IRX is off), charging the external capacitor to
• To switch to receive mode, IRX is turned on (ITX is off), increasing the voltage on the
• To switch to reverse mode, the current sources ITX, IRX are turned off, and the current
• To switch to idle mode, the current sources ITX, IRX, IFI are turned off, and the current
–240 mV below VM. (An internal clamp prevents further charging of the capacitor.)
capacitor to +240 mV with respect to VM.
source IFI is switched on, discharging the capacitor to VM.
source ISI charges the capacitor to VM.
– Both transmit level detectors sense higher signal levels than the respective receive
– The transmit background-noise monitor indicates the presence of speech
– Both receive level detectors sense higher signal levels than the respective transmit
– The receive background-noise monitor indicates the presence of speech
– The level detectors disagree on the relative strengths of the signal levels, and
– At least one of the background-noise monitors indicates speech
– Both speakers are quiet (no speech present), or
– When one speaker speech level is continuously overridden by noise at the other
level detectors, and
level detectors, and
speaker’s location
21. It operates as follows:
4872A–CORD–08/05
Figure