Service manual

2-4 June 21,2004

DECODER NP88 FLEX

Circuit Description

The decoder section consists of five functional blocks:

• Power Circuits

• Support Module

• Microcomputer (µC)

•LCD

• Alert Circuits

Refer to “Theory of Operation” in the GENERAL section for a functional block

diagram.

Power

Thenegativesideofthebatteryisc onnectedtocircuitcommon.Thepositivevoltage

from the battery is applied to the Support Module where it is converted to 3 V

operating voltage. The Support Module supplies the 3 Vdc to the µC and distrib-

utes the operating voltage throughout the decoder and receiver circuitry.

The memory backup battery ( BAT 1) is used to preserve the real-time clock

information and the contents of the message memory during primary battery

changes.Ifthebackupbatteryisfullycharged,allmemoryandtimedataisretained

in the pager for at least five minutes aft er the primary battery is removed. A good

primary battery must be installedforatleast 24 hours to fully recharge the backup

battery. The pager does not power up without a primary battery. The circuit ry

present in the pager prevents loss of memory provided the pager is off at the time

the primary battery is removed.

Support Module

The Support Module provides all voltage management, signal acquisition, and

decodersupportfunctions suchas alertgeneration,keypaddecoding,andcodeplug

retention. The Support Module consists of:

• Voltage management and support circuits that provide the main system volt-

ages for the decoder circuits.

• Voltage comparator circuits to detect the voltage of the primary battery and

provide feedback to the

µC

• Various signal acquisition circuits to decode and format the incoming audio sig-

nal from the receiver and buffer it for transfer to the

µC

• Clock Oscillator and support circuits to provide the main system clock of

38.4 kHz to the decoder and receiver.

• A keypad encoder to debounce the user-interface buttons.

• An alert generator to provide the main d riving waveform to the Transducer.

• A protected EEPROM array containing codeplug information.

Microcomputer

The Support Module initializes execution of the microcomputer (µC) program by

generating a reset pulse each time power is supplied to the pager.The µC decodes

the demodulated data from the receiver board, and sends the appropriate Trans-

ducer signal and message for the LCD to display.