MC33218ADWR2 Freescale Semiconductor, MC33218ADWR2 Datasheet - Page 17

• Manufacturer Part Number: MC33218ADWR2
• Description: IC VOICE SWITCH SPKPHONE 24-SOIC
• Manufacturer: Freescale Semiconductor

Specifications of MC33218ADWR2

• Function: Voice-Switched Speakerphone
• Interface: SPI Serial
• Voltage - Supply: 2.7 V ~ 6.5 V
• Current - Supply: 4.6mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: *
• Includes: Chip Disable
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power (watts): -
• Number Of Circuits: -
• Other names: MC33218ADWTR

Introduction
speakerphone and a telephone handset is that of half–duplex
versus full–duplex. The handset is full duplex, meaning
conversation can occur in both directions (transmit and
receive) simultaneously. This is possible due to both the low
sound level at the receiver, and the fact that the acoustic
coupling from the earpiece to the mouthpiece is almost
non–existent (the receiver is normally held against a person’s
ear). The loop gain from the receiver to the microphone and
through the circuit is well below that needed to sustain
oscillations.
in both the transmit and receive paths, and attempting to
converse full duplex results in oscillatory problems due to the
loop that exists within the speakerphone circuit. The loop is
formed by the hybrid, the acoustic coupling (speaker to
microphone), and the transmit and receive paths (between
the hybrid and the speaker/microphone). The only practical
and economical method used to date is to design the
speakerphone to function in a half duplex mode – i.e., only
one person speaks at a time, while the other listens. To
achieve this requires a circuit which can detect who is talking
(in reality, who is talking louder), switch “on” the appropriate
path (transmit or receive), and switch “off” (attenuate) the
other path. In this way, the loop gain is maintained less than
unity. When the talkers exchange function, the circuit must
quickly detect this, and switch the circuit appropriately. By
providing speech level detectors, the circuit operates in a
“hands–free” mode, eliminating the need for a “push–to–talk”
switch.
a voice switched, half duplex, speakerphone function. The IC
includes transmit and receive attenuators, pre–amplifiers, and
level detectors and background noise monitors for each path.
An attenuator control circuit automatically adjusts the gain of
the transmit and receive attenuators based on the relative
strengths of the voice signals present, the volume control, and
the supply voltage (when low). The detection sensitivity and
timing are externally controllable.
speakerphone integrated circuits in that it has a
microprocessor serial port for control of various functions.
Those functions are:
automatic switching.
the following sections.
Transmit and Receive Attenuators (Full Range B5 = 0)
performing a log–antilog function. When one is at maximum
gain (
(– 47 dB) – they are never both fully “on” or fully “off”. Both
attenuators are controlled by a single output from the
Attenuator Control Circuit which ensures the sum of their
gains will remain constant at a typical value of – 40 dB. Their
purpose is to provide the half–duplex operation required in a
speakerphone.
MOTOROLA ANALOG IC DEVICE DATA
The fundamental difference between the operation of a
A speakerphone, on the other hand, has higher gain levels
The MC33218A provides the necessary circuitry to perform
T h e M C 3 3 2 1 8 A i s u n i q u e c o m p a r e d t o m o s t
– Volume level (15 steps of
– Microphone amplifier mute
– Attenuator range selection (53 dB or 27 dB)
– Force to receive, idle, or transmit to override the
Please refer to the Block Diagram (Figure 3) when reading
The transmit and receive attenuators are complementary,
6.7 dB), the other is at maximum attenuation
3.0 dB each)
Freescale Semiconductor, Inc.
For More Information On This Product,
FUNCTIONAL DESCRIPTION
Go to: www.freescale.com
MC33218A
bandwidth of 50 kHz. Their input signal (at MCO and RXO)
should be limited to 300 mVrms (850 mVp–p) to prevent
distortion. That maximum recommended input signal is
independent of the volume control setting. Both the inputs and
outputs are biased at
until the output current limit (typically 2.0 mA peak) is reached.
Attenuator Control Circuit, which is measurable at C T (Pin 7).
When the circuit detects speech signals directing it to the
receive mode (by means of the level detectors described
below), an internal current source of 42 A will charge the C T
capacitor to a voltage positive with respect to V B (see
Figure 29). At the maximum volume control setting, this
voltage will be approximately +150 mV, and the receive
attenuator will have a gain of + 6.7 dB. When the circuit
detects speech signals directing it to the transmit mode, an
internal current source of 42 A will take the capacitor to
approximately – 100 mV with respect to V B (the transmit
attenuator will have a gain of + 6.7 dB). When there is no
speech present in either path, the current sources are shut
off, and the voltage at C T will decay to be equal to V B . This is
the idle mode, and the attenuators’ gains are nearly half–way
between their fully “on” and fully “off” positions (– 25 dB for
the R x attenuator, –16 dB for the T x attenuator). Monitoring
the C T voltage (with respect to V B ) is the most direct method
of monitoring the circuit’s mode, and its response.
Transmit and Receive Attenuators (Half Range B5 = 1)
which is “on” will have a gain of
attenuator will have a gain of
same as for the full range (– 25 dB for the R x attenuator,
–16 dB for the T x attenuator). The voltage at the C T pin, with
respect to V B, will be – 35 mV for the transmit mode, and
+ 85 mV for the receive mode.
Attenuator Control Circuit
are six: The T x
detectors, two background noise monitors, the AGC circuit,
the dial–tone detector, and the microprocessor interface.
These six functions are described as follows.
Level Detectors, T x
receive side and one on the transmit side (refer to Figure 30).
Each level detector is a high gain amplifier with
back–to–back diodes in the feedback path, resulting in
non–linear gain, which permits operation over a wide
dynamic range of speech levels. Refer to the graphs of
Figures 9, 10, and 11 for their DC and AC transfer
characteristics. The sensitivity of each level detector is
determined by the external resistor and capacitor at their
input (TLI and RLI). The output charges an external capacitor
through a diode and limiting resistor, thus providing a DC
representation of the input AC signal level. The outputs have
a quick rise time (determined by the capacitor and an internal
500
current source and the capacitor. The capacitors at RLO and
TLO should have the same value ( 10%) to prevent timing
problems.
The attenuators are non–inverting, and have a usable
The attenuators are controlled by the single output of the
With the attenuators set to the half range, the attenuator
The inputs to the Attenuator Control Section (Figure 29)
There are two identical level detectors – one on the
resistor), and a slow decay time set by an internal
–
R x comparator operated by the level
–
R x Comparator
V B . The output impedance is <10
– 34 dB. The idle mode is the
–7.0 dB, while the “off”
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