Theory Of Operation; Power Supplies; Keypad Keys; Status Led And Back Light Circuit - Motorola GM-950 Manual

Theory of Operation

3.0 Theory of Operation
3.1 General
The Control Head contains the internal speaker, the microphone connector, several buttons to
operate the radio, and several indicator Light Emitting Diodes (LED) to inform the user about the
radio status.
Additionally, the control head uses a 3 digit / 7 segment Liquid Crystal Display (LCD) for the
numerical information e.g. channel number, select code.
3.2

Power Supplies

The power supply to the Control Head is taken from the host radio's FLT A+ voltage via connector
J0901 pin 2 and the regulated +5V via connector J0901 pin 10. The voltage FLT A+ is at battery
level and is used for the LEDs, the back light and to power up the radio via the On / Off button. The
stabilized +5 volt is used for the display, the display driver, the shiftregister and the keypad buttons.
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 (D0925) and connects the base of (Q0925) to FLT A+. This transistor pulls the
line ANALOG 3 to low to inform the µ P that the On/Off button is pressed. If the radio is switched off,
the µ P 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.
3.4

Keypad Keys

The Control Head keypad is an (11) - key keypad. All keys are configured as 3 analogue lines
(ANALOG 1 2 3) to the radio. Lines ANALOG 1,2 each control four keys; line ANALOG 3 controls
three keys. The voltage on the analogue lines varies between 0V and +5V depending on which key
has been pressed. If a button is pressed, it will connect one of the 3 lines ANALOG 1,2,3 to a
resistive voltage divider connected to +5V. The voltages of the lines are A/D converted inside the µ P
and specify the pressed button.
3.5

Status LED and Back Light Circuit

All the indicator LEDs (4) and the back light LEDs (20) are driven by current sources (Q0942 -
Q0944, Q0951, Q0952) and controlled by the µ P via SERIAL PERIPHERAL INTERFACE (SPI). The
current is determined by the resistor at the emitter of the respective current source transistor.
Shiftregister (U0941) stores the LED status. To update the LED status line LED CLCK BUF shifts the
data of line SPI DATA BUF into the shiftregister. When all the data has been written, line LED CE is
set to low for a few microseconds to update the output of the shiftregister with the new data.
4B.1-2 Introduction/Theory of Operation