MT92303 Zarlink Semiconductor, MT92303 Datasheet - Page 8

• Manufacturer Part Number: MT92303
• Manufacturer: Zarlink Semiconductor

MT92303
Microphone Presence Detect
When an electret microphone is connected as shown
in Figures 8 and 9, it draws a bias current from the
MICBIAS pin. The voltage across the ‘bottom’ bias
resistor is monitored by the MICDETECT input and is
used to set a ‘presence’ flag in the MT92303’s Status
Register. This allows the external controller to
determine whether or not the handset and headset
are plugged-in. The internal workings of the
MT92303 are not affected by the state of the
‘presence’ flag.
Microphone Mute Detect
Any of the 4 Microphone inputs can be ‘muted’ under
direct software control via the ‘Mode Control’
register.
In addition, the MICDETECT inputs may also be
used to mute the microphone paths of the 2
associated
instantaneous blanking of the microphone path, and
consequently the sidetone path, and allows the ‘pop’
associated with electret muting to be avoided.
As shown in Figure 9, the internal ‘Mic Mute Detect’
signal is activated whenever the voltage across the
‘bottom’ bias resistor (R2) exceeds 40% of the
MICBIAS voltage. A small amount of hysteresis is
implemented such that the voltage needs to fall
8
Presence
Detect
Mute
Detect
Mic
Mic
Figure 9 - Mic Presence and Mute Detect
Audio
40%
35%
4%
3%
Interfaces.
MICDETECT
MICBIAS
in 15 steps
0 to 2.52V
This
GND
R1
R2
allows
below 35% of the MICBIAS voltage before the
internal ‘Mic Mute Detect’ signal is deactivated.
In addition to internally muting the microphone
channel, the ‘Mic Mute Detect’ signal is passed to the
MT92303’s Status Register.
The behaviour of the mute function can be
programmed to operate in four different ways.
It is important to note that in the third mode
(MUTEMODE=10) described in table 2, it is the
designer’s responsibility to re-enable the microphone
path after it has been muted by the MICDETECT pin.
This allows the designer to re-establish the signal
path after a time sufficient for the settling of any
microphone bias circuitry.
The on-chip ‘Status’ register provides access to the
internal ‘Mic Mute Detect’ signal, and also the
acknowledged version ‘Mute’ which denotes whether
or not the microphone channel is actually muted.
These two bits of information are expected to be
used by the external controller to determine when
the channel needs to be re-enabled. This will be
when ‘Mute’ is high, and ‘Mic Mute Detect’ has fallen
low, but the external controller may introduce a delay
before re-establishing the microphone path. The path
should be re-enabled by momentarily switching to
MUTEMODE=00.
Bandgap Voltage Reference
The MT92303 has its own internal bandgap voltage
reference which determines the absolute gains of the
TX & RX paths. The bandgap voltage is brought out
to the VREF pin for external decoupling with a
capacitor (100nF to GNDA) that should be located as
MODE
MUTE
00
01
10
11
Table 2. Microphone Mute Detection
Microphone path enabled (normal
operation). ‘Mic Mute Detect’ signal
ignored.
Microphone path disabled (muted) while
‘Mic Mute Detect’ signal is high, and
enabled while low.
Microphone path disabled (muted) while
‘Mic Mute Detect’ signal is high, but must
be re-enabled by the external
controller .
Microphone path disabled (muted). ‘Mic
Mute Detect’ signal ignored.
Mute Operation (see Figure 9)
Preliminary Information