DESIGNING THE ISD33000 SERIES INTO DIGITAL CELLULAR PHONES
Introduction
Cellular phone manufacturers are constantly under pressure to release new models of
phones with compact designs and more user features. This puts tremendous pressure on
their development teams to re-engineer the phone architecture, software development,
hardware development and layout. Time to market with a new product is also crucial
for manufacturers to remain competitive.
An emerging trend in cellular phone handsets is the addition of “voice” features. These
voice features include: 1) the “on-the-fly” recording of telephone conversations, which
eliminates the need for writing down notes and improves driving safety by enabling
hands free usage, 2) recording voice memos, where reminders are left for oneself, and
3) integrating a telephone answering machine, which can screen or capture otherwise
missed calls. The basic function which enables these new features is the ability to reli-
ably record and playback voice messages. To meet the requirements of cellular handset
performance, the solution must be low-power, to conserve battery life, have a small form
factor, to enable a compact design, low-cost to keep the retail pricing down, and repro-
duce high quality sound, to meet the expectations of consumers. Additional factors to
consider are development time and costs, since a quick time-to-market and effective
design investment are critical for new products.
The following design illustrates how a single ISD33000 device can be integrated into a
GSM cellular handset to enable these new “voice” features. With minimal hardware and
software development, this solution provides one to four minutes of high quality, voice
record and playback capability with low-power consumption and minimal board space.
In addition, this implementation of the ISD33000 is easily portable to other analog or
digital cellular, PCS or PHS handsets because it is integrated in a manner which is inde-
pendent of the transmission and processing protocol being utilized.
Integration of an ISD33000 into a GSM Phone
The block diagram of the ISD33000 series voice recorder chip integrated into a typical
GSM digital cellular phone is shown in Figure 1. The GSM architecture is based on a
typical GSM chip-set available from one of the several semiconductor manufacturers in
the U.S.A.
Figure 1
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GSM
RF
Section
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GSM
Baseband
Section
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The ISD33000 series chip interfaces directly to the earpiece and microphone signals
without involving the DSP software for speech encoding and decoding or adding code
to the protocol stack, layer 1, layer 2 and layer 3 of the GSM protocol. The ISD33000
series is also connected directly to the system micro-controller’s standard 8-bit general
purpose I/O port. This connection provides the user interface control and message
management.
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System
Microcontroller,
Memory,
Keyboard
Display and Battery
Management
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ISD33000
Series
Voice Recorder
+
Audio Amplifier
and Switches
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RF Section
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GSM
RF
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GP1O0
GP1O1
GP1O2
GP1O3
GP1O4
GP1O5
GP1O6
GP1O7
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System
section
Microcontroller
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TDA7050T
The detailed schematic of the ISD33000 series application is as shown in Figure 2.
Besides the voice recorder chip, only two other ICs are needed. Switches S1, S2 and S3
are implemented by using a 74HC4053 as an analog multiplexer switch to select various
audio input and output paths. This comes in a single 16-pin SOIC package.
TDA7050T, which is packaged in a small 8-pin SOIC, is an integrated audio amplifier
with fixed gain to drive the earpiece. A couple of capacitors are used for blocking the
DC voltages in several sections of the design as shown.
The general purpose I/O lines on the system microcontroller are used to control and
interface with the ISD33000 and the 74HC4053 switches. GPIO0 controls the switch-
es S1 and S2 and does the source selection between the audio input / output of the
ISD33000 and the GSM baseband section. Switch S3 is controlled by GPIO1 and mul-
tiplexes the audio output of the GSM baseband and the ISD33000 to the earpiece.
Resistors R1 and R2 provide attenuation to the signal from the GSM baseband to the
analog input of the ISD33000. R3 is used to attenuate signals going to the fixed gain
audio amplifier TDA7050T. GPIO2:7 are used to interface with the ISD33000’s con-
trol signals RAC, /INT, /SS, SCLK, MOSI and MISO. (Note: more information about
these signals and their functions can be obtained from the data sheet for the ISD33000.)
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Audio
OUT
R1
R2
TDA7050T
Microphone
S2
Earpiece
ISD33000
Series
GSM
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GSM
Baseband
Section
Audio
IN
S1
Figure 2
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S3
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ANA IN-
ANA+
AUD OUT
R3
RAC
INT
SS
SCLK
MOSI
MISO
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