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THEORY OF OPERATION (CONTROL HEADS)
Controller
LCD controller U2 contains the default character set. The controller is expanded by use of an LCD
driver, U3. The LCD controller is connected the µC via data bus PA0-PA7, while the control and
read/write (R/W) lines are connected via PB5 and PB7 respectively.
Power Supplies
The LCD bias voltage drives the display segments. The bias voltage is divided into five levels by
resistors R52 through R56, and routed to the LCD controller and LCD driver under µC control via
U10. The µC measures the bias voltage with respect to Vcc (TP3) at A/D port PD3.
Another function of the bias voltage is temperature, which is measured across R2 at A/D port PD7.
The µC calculates the correction to the bias voltage and adjusts the duty cycle of the PLMA output
to U10 accordingly.
Temperature Sensor
µC U4 measures the voltage on PD7, which is controlled by dividers R2 and R8. R8 is an NTC resis-
tor that is placed in close proximity to the LCD display. The temperature sensor protects the back-
light LEDs from failure, and is also used to compensate the LCD bias voltage, dependent on the
LCD glass temperature. The protection temperature is 77º±6ºC.
Keypad
The keypad is a passive input-scanning matrix, with each key having three poles. When a key is
pressed, a row and a column are connected to ground and sensed by the µC (PB0-PB4 and PC0-
PC4).
Backlight
The backlight consists of 13 LEDs. It can be switched on/off by the radio software, controlled by the
µC, TCMP1 and TCMP2. A high pulse on TCMP1 turns the backlight on, and a low pulse on TCMP2
turns it off. If the temperature in the Control Head rises above 78ºC, the µC turns the backlight off,
using the Temperature Sensor input.
External Microphones and Speakers
External microphones and speakers are connected via the transceiver's rear accessory connector
Helmet
A helmet including a microphone, speaker and if required a PTT is connected via the 10pin TELCO
connector at the EXPANSION HEAD.
MTM700 Mobile Radio / Detailed Service Manual
4.3 - 9
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Troubleshooting
