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CUC 7303 F

Description des circuits / Circuit Description

2. System Control

2.1 Microcomputer
The mask-programmed 8-bit Microcomputer IC850 (SDA5222 without
Text) decodes the commands entered on the keyboard and also the
infra-red remote control commands from the IR-receiver. It is also
responsible for the total system control and the on-screen display
(OSD). All data for the programme positions and the options are stored
in the NVM (Non Volatile Memory). The teletext (Videotext) IC is
integrated in SDA5252.

The correct operation of the microcomputer depends on the following
conditions:
- Supply v5V/H at Pin 37
- Oscillator frequency 18MHz at Pins 12, 13
- Reset pulse:

Every time the TV receiver is switched on with the mains button, the
processor is reset on Pin 1 by the reset pulse.

- I

C-bus:

The I

C-bus is a bidirectional two-lead bus consisting of the SDA

(System Data) lead and the SCL (System Clock) lead.

Checking the operation of the processor IC850:
The I

C-bus leads are connected via the pull-up resistors CR869 and

CR868 to +5V/H. The data traffic is controlled from the processor which
also generates the SCL bus clock. The only way to check the data and
clock leads when servicing is by measuring the TTL-levels (L

≤

0.8V;

≥

3.5V).

Service note:
The I

C-bus data is also present without a command from the IR

remote control handset. If no data is carried on the bus leads there may
be a short circuit. To localize the fault, the modules and components
connected to the data bus must be unsoldered or unplugged one after
the other.

2.2 Initialisation of the Processor after Switching On
When the TV is switched on, the +5V/H voltage builds up, the
IC850-(15) is reset, and the programme sequence is started.
With the start command, the processor feeds out a "High" level at
Pin 40 and the voltage U

Standby

starts the TV via CT826, IC676-(1) by

means of the voB, 12V (see Power Supply).

After switching on, the processor (IC850) transfers the operating data
from the internal memory via the I

C-bus to the bus-controlled modules

and circuits.

2.3 Switching over of the CCVS Signals to the Scart Socket
A "High"-level switching voltage U

FBAS

at IC850-(16) causes the CCVS

signal FBAS

to be switched to output pin 19 of the Scart socket.

2.4 Entering Commands
The keyboard is connected to the unswitched v5V/H. By
evaluating the different voltage levels, the processor IC850-(27),-(28)
knows which button on the keyboard has been pressed.
The remote control commands are amplified by the infrared receiver
IC810 and decoded at Pin 8 of the microprocessor.

2.5 Teletext IC850 (SDA5252)
The IC850 (SDA5252) contains a 1-page Teletext-IC. The On Screen
Display is subdivided into lines and columns. For positioning and
synchronising the teletext display, horizontal and vertical reference
pulses are fed to IC850-(45, 46). Activation of the teletext is effected
internally via the I

C-bus. Via pin 30, the SDA5252 scans the CCVS

signal for teletext data.

2.6 On Screen Display (OSD)
For displaying data on the screen, the switching voltage "U

Data

IC850-(50) supplies a "High" level switching IC150-(21)

≤

2V to RGB

mode. The character generator feeds out the display data via the
output ports 47, 48, 49 of the microprocessor at an amplitude of 4.5V
to the RGB inputs IC150-(22), -(23), -(24), approx. 450mV.

2.7 Protection Circuit U

Schutz

The base of the transistor T511 is connected via R511 to the low-end
point of the vertical output stage, and via R512, D512, D513 to the
reference pulse F from the horizontal output stage. In the case of any
failure, a base voltage of 0.6V and higher switches the transistor on; via
its collector the transistor switches IC850-(32) to ground. The

P then

switches the TV to standby.
If the vD fails there is no direct voltage present at the output of
the vertical output stage IC400-(5) and consequently the proctection
circuit input IC850-(32) is pulled to ground.
At the same time the collector (SB lead) is connected via R513, D514,
and CD516 to the low-end point of the high-tension winding. When the
Zener voltage is exceeded due to too high a beam current the collector
voltage decreases to 0V so that the TV switches to standby.

UC 3842A

Internal

Bias

Reference

Regulator

Oscillator

1.0mA

Error

Amplifier

1.0V

Current Sense
Compartator

Gnd