Motorola GM338, Service Manual

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Controlhead Model for GM338 2-7
3.7 Liquid Crystal Display (LCD)
The LCD H0971 uses the display driver U0971. The display is a single layer super twist nematic
(STN) LCD display. It has 14 characters with a 5*8 dot matrix for displaying alpha - numerical
information and a line with 21 pre - defined icons above the dot matrix
The driver contains a data interface to the µP, an LCD segment driver, an LCD power circuit, an
oscillator, data RAM and control logic. At power up the driver’s control logic is reset by a logic “0” at
input SR2 (U0971-15). The driver’s internal oscillator is set to about 20 kHz and can be measured at
pin 22. The driver’s µP interface is configured to accept 8 bit parallel data input (U0971-D0-D7) from
the controlhead µP (U0831 port PC0-PC7).
To write data to the driver’s RAM the µP sets chip select (U0971-20) to logic “0” via U0831-11, RD
(U0971-18) to logic “1” via (U0831-10) and WR (U0971-17) to logic “0” via U0831-9. With input A0
(U0971-21) set to logic “0” via U0831-12 the µP writes control data to the driver. Control data
includes the RAM start address for the following display data. With input A0 set to logic “1” the µP
then writes the display data to the display RAM. When data transfer is complete the µP terminates
the chip select, RD and WD activities.
The display driver’s power circuit provides the voltage supply for the display. This circuit consists of
a voltage multiplier, voltage regulator and a voltage follower. The external capacitors C0971 -
C0973 configure the multiplier to double the supply voltage. In this configuration the multiplier output
VOUT (U0971-8) supplies a voltage of -5V (2* -5V below VDD). The multiplied voltage VOUT is sent
to the internal voltage regulator. To set the voltage level of the regulator output V5 (U0971-5) this
voltage is divided by the resistors R0973 and R0974 and fed back to the reference input VR (U0971-
6). In addition the regulator output voltage V5 can be controlled electronically by a control command
sent to the driver. With the used configuration the voltage V5 is about –2V. The voltage V5 is
resistively divided by the driver’s voltage follower to provide the voltages V1 - V4. These voltages
are needed for driving the liquid crystals. The level of V5 can be measured by one of the µP’s
analogue to digital converters (U0831-20) via resistive divider R0975, R0976. To stabilize the
display brightness over a large temperature range the µP measures the temperature via analogue to
digital converter (U0831-18) using temperature sensor U0834. Dependent on the measured
temperature the µP adjusts the driver output voltage V5, and in turn the display brightness, via
parallel interface.
3.8 Microphone Connector Signals
Signals BUS+, PTT IRDEC, HOOK, MIC, HANDSET AUDIO, FLT A+, +5V and 2 A/D converter
inputs are available at the microphone connector J0811. Signal BUS+ (J0811-7) connects to the
SBEP bus for communication with the CPS or the Universal Tuner. Line MIC (J0811-5) feeds the
audio from the microphone to the radio’s controller via connector J0801-4. Line HANDSET AUDIO
(J0811-8) feeds the receiver audio from the controller (J0801-6) to a connected handset. FLT A+,
which is at supply voltage level, and +5V are used to supply any connected accessory like a
microphone or a handset.
The 2 A/D converter inputs (J0811-9/10) are used for a microphone with keypad. A pressed key will
change the dc voltage on both lines. The voltages depend on which key is pressed. The µP
determines from the voltage on these lines which key is pressed and sends the information to the
host radio.