U4091BM-N Datasheet, PDF (16/38 Pages) ATMEL Corporation – PROGRAMMABLE TELEPHONE AUDIO PROCESSOR

Power-on Reset
Watchdog Function
Acoustic Feedback
Suppression
To avoid undefined states of the system when it is powered on, an internal reset clears
the internal registers.
The system (U4091BM-N + microcontroller) is woken up by any of the following
conditions:
• VMP > 2.75 V and VB > 2.95 V
• and line voltage (VL)
• or ringer (VRING)
• or external supply (ES)
The power-down of the circuit is caused by a shut-down sent by the serial bus (SD = 1),
low-voltage reset or by the watchdog function (see Figure 9 on page 17, Figure 10 on
page 18 and Figure 11 on page 18).
To avoid the system operating the microcontroller in a wrong condition, the circuit pro-
vides a watchdog function. The watchdog has to be retriggered every second by
triggering the serial bus (sending information to the IC or other remote components at
the serial bus). If there has been no bus transmission for more than one second, the
watchdog initiates a reset.
The watchdog provides a reset for the external microcontroller, but does not change the
U4091BM-N's registers.
Acoustical feedback from the loudspeaker to the hands-free microphone may cause
instability of the system. The U4091BM-N has a very efficient feedback-suppression
circuit which offers a 4-point- or alternatively a 2-point-signal-sensing topology (see Fig-
ure 8 on page 17).
Two attenuators (TXA and SAI) reduce the critical loop gain via the serial bus either in
the transmit or in the receive path. The overall loop gain remains constant under all
operating conditions.
The LOGs produce a logarithmically-compressed signal of the TX- and RX-envelope
curve. The AFSCON block determines whether the TX or the RX signal has to be
attenuated.
The voice-switch topology can be selected by the serial bus. In 2-point-sensing mode,
AFSCON is controlled directly by the LOG outputs.
16 U4091BM-N
4666B–CORD–08/04