Status Led And Back Light Circuit; Liquid Crystal Display (Lcd) - Motorola CDM Series Service Manual

Chapter 2: 2-6
3.6

Status LED and Back Light Circuit

All indicator LEDs (red, yellow, and green) are driven by current sources. To change the LED status
the host radio sends a data message via SBEP bus to the control head µP. The control head µP
determines the LED status based 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, therefore, 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 causes a proportional voltage drop across the parallel
resistors R0947 and 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 and
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.
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 and a 5x8 dot matrix to display alphanumerical 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 control head 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 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 then 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 required to drive the liquid crystals. The level of V5 can be measured by one of the µP's
analog-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 analog-to-digital
6881091C63-A
Control Head (CDM1250 / PRO5100 / CDM1550 Series / PRO7100)