Motorola GM1200E Service Manual Download

Page: 22 / 104

Theory of Operation

2.1-6

Introduction/Theory of Operation

The display driver is fixed on the flex which connects the display to the PC board. The driver
contains a data interface to the

P, 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 RES
(U0902-9). Resistor R0946 sets the driver’s internal oscillator to about 18 kHz. By connecting U0902
pin 12 to +5V the driver’s

P interface is configured to accept 8 bit parallel data input (U0902-D0-D7)

from the control head

P (U0901 port PC0-PC7). Pin 15 connected to +5V sets the 6800

P control

mode.

To write data to the driver’s RAM the

P sets chip select (U0902-14) to logic ‘1’ via U0901-59 and R/

W (U0902-17) to logic ‘0’ via U0901-56. With input A0 (U0902-16) set to logic ‘0’ via U0901-58 the

P writes control data to the driver. Clock signal E at pin 18 generated by

P pin 57, shifts 8 bit

parallel data into the driver. Control data includes the RAM start address for the following display
data. With input A0 set to logic ‘1’ the

P then writes the display data to the display RAM. When data

transfer is complete the

P terminates the chip select and the clock activities.

The voltage supply for the display is provided by the display driver power circuit. This circuit consists
of a voltage multiplier, voltage regulator and a voltage follower. To use an external voltage supply the
built-in power circuit can be turned off by a control command. The settings of the inputs T1 (U0902-
36) and T2 (U0902-35) select among the various functions of the power circuit. With both inputs set
to ground level by resistors R0955 and R0956 the voltage multiplier, the voltage regulator and the
voltage follower are activated and no external voltage supply is required for the LCD. The external
capacitors C0951 - C0953 configure the multiplier to triple the supply voltage. If R0957 is used
instead of C0952 the multiplier doubles the supply voltage. In this configuration the multiplier output
VOUT (U0902-42) 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 (U0902-43)
this voltage is divided by the resistors R0958 and R0959 and feed back to the reference input VR
(U0902-44). 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 -3V. 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 driver circuit can be configured to use externally
generated voltages for VOUT and V1 - V5. In this case the +5V supply voltage is multiplied by the

(U0901-62) along with the multiplier circuit D0911, C0911, C0912, R0911 and R0913. The

provides a square wave signal at pin 62 to drive the multiplier circuit. The voltages V1-V4 are
generated from VOUT or V5 by the resistive divider R0941 - R0945 and supplied to the driver ports
V1 - V5. Dependent on the configuration the level of VOUT or V5 can be measured by one of the

P’s analogue to digital converters (U0901-20) via resistive divider R0914, R0915. To stabilize the

display brightness over a large temperature range the

P measures the temperature via analogue to

digital converter (U0901-18) using thermistor R0918 and resistor R0917. Dependent on the
measured temperature the

P adjusts the driver output voltage V5, and in turn the display

brightness, via parallel interface.

3.10

Microphone Connector

Signals BUS+, PTT, HOOK, MIC HI, HANDSET AUDIO and FLT A+ available at the microphone
connector J0903, are connected to the radio’s controller section via connector J0901.

3.11

Electrostatic Transient Protection

Electrostatic transient protection is provided for the sensitive components in the control head by
diodes VR0901 - VR0905, VR0931 - VR0935. The diodes limit any transient voltages to tolerable
levels. The associated capacitors provide Radio Frequency Interference (RFI) protection.