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3.5.2
Data Communications
The data is transmitted between the handset and base at 625 baud using Minimum Shift Keying (MSK) format. The two frequencies
used in the keying are 300Hz and 600Hz which represents logic 0 and logic 1 respectively. A separate modem chip is not required in
the design since the MCU generates and decodes all the data. Details of the software communications protocol are not discussed in
this document but are available upon request,
The level for data transmissions are set to produce about +/- 40kHz of deviation. This level was determined empirically to provide
optimum data sensitivity. In the idle state, the transmit data port on the MCU is set to high impedance.
Received data from the handset is passed from the demodulator IC to the buffer amplifier where it undergoes lowpass filtering. From
the buffer, the receive data is split off to the receive data chain; as mentioned in a previous section, the buffer amplifier is shared by
both the audio and data, thus requiring a splitting junction at the buffer output. This receive data chain consists of lowpass filtering
and a comparator to restore the data to its original condition. After conditioning, the data is coupled directly to the MCU for analysis.
3.5.3
Clock Reference
The MCU crystal frequency is 4MHz and is shared with the RF module where it is used as a reference for the phase lock loop (PLL).
A trimmer capacitor is connected on one end of the crystal and is used to pull the frequency into specification. The trimmer capacitor
compensates for component tolerances, crystal variations and any other parasitics that may affect the oscillating frequency. The PLL
reference is tapped off the MCU’s oscillator circuit via a buffer amplifier to prevent loading of the oscillator circuit.
3.5.4
Reset Circuit
The reset circuit for the MCU consists of a reset IC and supporting components. The IC is designed to reset the MCU when the power
supply drops to about 4.6VDC and below. This insures that if the power supply drops to a level where logic levels may become
indeterminate, the MCU will be reset to a known condition, potentially preventing erronceus operation. In addition to being con-
nected to the reset IC, the MCU’s active low reset line is connected to the power rail via an RC network. This RC network ensures
that after a reset, the MCU’s reset line is brought back up to a logic high cleanly and continuously.
3.5.5
EEPROM
An EEPROM (NM93C46EM8) is used to store speed dial numbers, the default channel and the security code so that they are not lost
in the event of power failure.
3.5.6
ATE Interface
ATE test points are available on the base to facilitate ATE testing. The ATE uses these test points to access the signals required to
complete a base alignment. Base to ATE communication is accomplished through a dedicated port on the MCU which is connected
directly to a charge contact. As with other charge contacts the ATE I/O contacts are protected from ESD using LC filtering and
additional protection is afforded at the MCU’s port by using protection diodes.
3.5.7
LED Expansion Connector
Therefore, to facilitate changes in phone cosmetics for applications without having multiple base versions, an expansion connector
which provides remote access to the LED activating circuits and the page line is provided. This connector allows other vendors to
specify different LED and the page key locations on the base.
HANDSET SECTION
4.1 Introduction
This Machine is a 900MHz analog cordless telephone. The phone was developed with low cost and high performance in mind. The phone
minimizes costs by using analog speech transmission and by utilizing a number of new design innovations that reduce production costs. The
speech transmission differs from our digital phones in that digital speech encoding and decoding does not occur in speech transmissions. This
means that elaborate software algorithms and additional hardware are avoided in the design. This document describes the technical aspects
and operation of this Machine handset.
All RF, audio, MCU control and power management components are placed on a single PCB. The use of a single PC board reduces costs
associated with ATE testing, handling and assembling.
The keypad used in this design is also a first and is printed on a separate mylar sheet that is attached to the back of the PC board with the
sheet’s own adhesive. Using this type of keypad prevents us from having to re-layout the PC board in the event that a different keypad
arrangement is requested by one.
This Machine handset uses a three cell battery which is also a first. The smaller and lighter battery reduces costs while improving the
esthetics of the handset by making it more compact. To further facilitate a smaller and lighter handset, most of the discrete components are
surface mount and use formats such as those in the BP-T23.
4.2 Power Management
This Machine handset power is supplied by a three cell battery with a nominal voltage of about 3.6VDC. This voltage is then regulated to 3
VDC and distributed throughout the handset circuits. The handset ringer is the only circuit that operates directly off the handset battery.
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