Theory Of Operation; General; Power Supplies; Power On / Off - Motorola GM-950 Manual

Theory of Operation

3.0 Theory of Operation
3.1

General

The Control Microphone contains the speaker, the microphone, several buttons to operate the radio,
several indicator Light Emitting Diodes (LED) to inform the user about the radio status and a 3 digit /
7 segment Liquid Crystal Display (LCD) for nummerical information e.g. channel number, select
code.
3.2

Power Supplies

The power supply to the HHCM is taken from the host radio's FLT A+ voltage via connector J0901
pin 8. This voltage is at battery level and is used for the voltage regulator (U0971), the LEDs, the
back light and to power up the radio via the On / Off button. The voltage regulator (U0971) provides
the stabilized 5 volt source for the display, the display driver, the shiftregister and the keypad buttons.
The regulated output of U0971 is enabled by a low state and disabled by a high state at pin 3 to
switch the HHCM circuits on and off. Input and output capacitors (C0972 and C0975, C0976) are
used to reduce high frequency noise and provide proper operation during battery transients.
3.3

Power On / Off

The On/Off button when pressed switches the radio's voltage regulators on by pulling ON OFF
CONTROL to high via D0971 and connects the base of Q0971 to FLT A+. This transistor pulls the
line ANALOG 3 to low to inform the µP that the On/Off button is pressed. If the radio is switched off,
the µP will switch it on and vice versa. If the On/Off button is pressed and held while the radio is on,
the software detects a low state on line ANALOG 3 and switches the radio off.
While the radio is switched on, the radio's controller enables the regulated output pin 1 of U0971 by
setting line LED CE to high and in turn pulling input U0971-3 to ground by transistor Q0972. Line
LED CE is also used to write data into the shiftregister U0983. After the serial write process line LED
CE is set to low for a few microseconds to update the output of the shiftregister with the new data.
The low - pass filtering provided by C0973 and R0974 filters out these small low periods and
prevents disabling of the voltage regulator output.
3.4

PTT Button

The PTT function is achieved by pressing button PB0901 which places a low on the PTT line to the
radio, causing the radio to transmit.
3.5

ON/OFF Hook Control

When the HHCM is off-hook the monitor input (line HOOK; J0901-9) to the radio is open circuit.
When the HHCM is placed on-hook the hang up clip shorts the TP0901 and TP0902 terminals which
takes the monitor input to ground, indicating to the radio that the microphone is on-hook. Diode
VR0922 and C0922, C0923 provide Electro Static Discharge (ESD) and Radio Frequency
Interference (RFI) protection.
4C.1-2 Introduction/Theory of Operation