Keypad Keys; Status Led And Back Light Circuit - Motorola GM328 Service Manual

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When the controlhead µP wants to communicate to the host radio, the µP brings request line CH
REQUEST to a logic "0" via µP pin 62. This switches on Q0823, which pulls line ON OFF
CONTROL high through diode D0821. A low to high transition on this line informs the radio, that the
controlhead requires service. The host radio then sends a data request message via BUS+ and the
controlhead µP replies with the data it wanted to send. This data can be information like which key
has been pressed or that the volume knob has been rotated.
The controlhead µP monitors all messages sent via BUS+, but ignores any data communication
between host radio and CPS or Universal Tuner.
3.5

Keypad Keys

The controlhead keypad is a 6-key kepad (Model B) or a 10- key keypad (model C). All keys are
configured as 2 analogue lines read by µP pins 13 and 15. The voltage on the analogue lines varies
between 0 volts and +5 volts depending on which key has been pressed. If no key is pressed, the
voltage at both lines will be 5 volts. The key configuration can be thought of as a matrix, where the
two lines represent one row and one column. Each line is connected to a resistive divider powered
by +5 volts. If a button is pressed, it will connect one specific resistor of each divider line to ground
level and thereby reduce the voltages on the analogue lines The voltages of the lines are A/D
converted inside the µP (ports PE 0 - 1) and specify the pressed button. To determine which key is
pressed, the voltage of both lines must be considered.
An additional pair of analogue lines and A/D µP ports (PE 3 – 2) is available to support a keypad
microphone, connected to the microphone connector J0811. Any microphone key press is
processed the same way as a key press on the controlhead.
3.6

Status LED and Back Light Circuit

All the indicator LEDs (red, yellow, green) are driven by current sources. To change the LED status
the host radio sends a data message via SBEP bus to the controlhead µP . The controlhead µP
determines the LED status from the received message and switches the LEDs on or off via port PB
7 – 0 and port PA4. The LED status is stored in the µP 's memory. The LED current is determined by
the resistor at the emitter of the respective current source transistor.
The back light for the LCD and the keypad is controlled by the host radio the same way as the
indicator LEDs using µP port PA 5. This port is a Pulse Width Modulator (PWM) output. The output
signal charges capacitor C0843 through R0847. By changing the pulse width under software
control, the dc voltage of C0843 and thereby, the brightness of the back light can be changed in four
steps. The keypad back light current is drawn from the FLT A+ source and controlled by transistor
Q0933. The current flowing through the LEDs cause a proportional voltage drop across the parallel
resistors R0947, R0948. This voltage drop is amplified by the op-amp U0931-2. U0931-2 and
Q0934 form a differential amplifier. The voltage difference between the base of Q0934 and the
output of U0931-2 determines the current from the base of the LED control transistor Q0933 and in
turn the brightness of the LEDs. The µP can control the LEDs by changing the dc level at the base
of Q0934. If the base of Q0934 is at ground level, Q0934 is switched off and no current flows
through Q0933 and the LEDs. If the base voltage of Q0934 rises a current flows through Q0934 and
in turn through Q0933 causing the LEDs to turn on and a rising voltage drop across R0947, R0948.
The rising voltage causes the output of the op-amp to rise and to reduce the base to emitter voltage
of Q0934. This decreases the current of Q0933 until the loop has settled.
Controlhead Model for GM338