December 6, 2004 68P81083C20-D
8-2 Control Heads: Model I
LCD
Backlight
The backlight consists of 10 LEDs. Light intensity is in 3 levels including off and is software controlled
by bus commands via µC U0653.
Driver
The LCD driver U0651 contains the default character set. Its interface is a four line serial interface.
The LCD driver is connected to the µC on pins PA0 - PA3. The driver has a built in oscillator with one
external resistor, R0700, defining the clock frequency. The oscillator runs at approx. 25 kHz giving an
LCD frame frequency of approx. 48 Hz.
Power Supplies
The LCD driver and bias voltages are powered either from 5.0 V or 5.7 V. The LCD bias voltage
levels are created by the voltage divider R0701, R0702 and R0703. This gives four voltages: supply
voltage (VDD), V1, V2 and V3=GND. The COM and SEG outputs from U651 jump between these 4
bias voltage levels, which create the on and off AC voltage for each LCD segment.
Resistors R0715 and R0716 prevent U0651 from going into test mode during power up.
The LCD supply voltage is controlled by the microcontroller U0653 PA6.
Display
Display (H0650) is a transmissive/reflective positive LCD display. The Control Head has one line of 8
characters with fixed symbols on top, four key-related symbols on the bottom and three color
indicators.
Color Indicators
The three color indicators are 4 LEDs D0679, D0680, D0681 and D0682. There are two yellow LEDs
in order to increase the color intensity.
Keypad
Backlight
The keypad backlight is turned on and controlled along with the LCD backlight.
Keypad
The keypad is a static input with 10 kohms pull-up resistors. There is no scanning, but simply one
input for each key. Debouncing control is handled by the software.
Mode Select
To reduce the number of pins on the microphone connector multiplexing is utilized to switch between
the 3 different modes, Normal mode, SB9600 mode and Keyload mode.
Multiplexer Operation
The multiplexer comprises U0655 and U0656. Inputs to the multiplexer are from the microphone
connector pins 3, 6 and 7. The multiplexer is switched either by a current sensing device (Q0662,
Q0663), for Normal and SB9600 Bus modes, or by software for keyload mode. Pin 1 of the
microphone connector carries the output current of the connected device.
When the output current is below 30 µA, the multiplexer is switched to the Normal mode.
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