Hitachi C1422T Wartungshandbuch Seite 21

Microprocessor Control System
Microcontroller
Micro-controller, I701, is either a SAA5288 (on non-teletext sets) or a SAA5290 (on teletext sets). Both
devices have integrated on-screen display (OSD) generator; the SAA5290 also has an integrated teletext
decoder. The micro-controller has a single 12MHz crystal X701.
The television is controlled by serial (I²C) bus communication, digital switching inputs and outputs, analogue-
to-digital inputs and pulse-width-modulation (PWM) outputs.
Reset
At power on, C709 is not charged so the voltage on pin 43 of the micro-controller rises with the 5V supply
and resets the micro-controller. An internal resistor to ground at this pin causes C709 to charge up and the
voltage on the reset pin to drop to 0V.
In normal operation, transistor Q702 is switched on and the reset pin is low, but if the 10V supply voltage
drops below about 8V the transistor will switch off and R721 will pull the reset pin high and reset the device.
User Control
User input is via infra-red remote control (Philips' RC5 protocol), internally decoded from the receiver I703, or
from three (non-matrixed) local control keys on active low inputs, I701 pins 18, 19 and 20. The LED flashes
each time a key is detected or remote command received.
Tuning
The tuning control voltage to the tuner is controlled via the PWM at pin 1 of I701 and integrating circuit
around Q001. Minimum voltage is at maximum mark-space ratio (bottom of each band). Band-switching is
controlled by active low outputs on pins 14, 15 and 16 (high, mid, low) and transistors Q002 to Q004. The
controller makes AFC corrections by reading on-tune information from I501 via the I²C bus.
As the tuner is controlled by voltage synthesis, there is no direct correlation between the controller output
and the tuned frequency.
Non-volatile memory
The non-volatile memory, I702, holds configuration information, user settings, parameters as applicable for
I²C controlled ICs and the programme tuning records. It is itself accessed by I²C.
When a new memory IC is fitted, the microcontroller will automatically load default information, which takes a
few seconds during power-up. Regular re-loading of the data, or corruption of settings may indicate I702 is
faulty.
AV switching
Inputs from pins 8 and 16 of SCART 1 are sampled by analogue to digital conversion on pins 9 and 10 of
I701 respectively. The input levels are adjusted such that the controller will automatically switch to AV1 when
pin 8 is above 6V and RGB when pin 16 is above 1V. The time constant on pin 16 ensures the controller will
not detect real-time RGB insertion using pin 16.
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