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CPU RESET OPERATION
The CPU must be reset each time AC power is applied. The reset function
ensures that the 5 volt power supply is supplying sufficient power to the CPU,
and the crystal-controlled reference oscillator has stabilized before the CPU
may detect inputs from the keyboard or remote control. The reset operation
will also cancel any programs operating before the power was removed. The
circuitry to reset the CPU consists of Q831 and associated components.
After the reset function is complete, the following CPU conditions will exist:
A. Pin 27 (power) will be maintained in a Low state (TV power supply
Off).
B. The digital control functions (brightness, contrast, etc.) will be set to
the last setting modes (FACTORY PRESET or MANUAL).
C. The BUS data (sub color, sub tint, etc.) will be set to the last settings.
D. The time of day clock will be reset to zero and all Timer functions
cleared.
E. The volume control output is set to the last setting level.
F. The channel selection will be set to last channel selected.
G. The TV/AV mode will be set to the last TV/AV mode selected.
H. The MTS mode will be reset to Stereo.
I.
The Surround mode will be set to the last Surround mode (ON or OFF)
selected.
J. The PIP mode will be set to OFF.
K. The Caption mode will be set to the last Caption mode selected.
L. The Language mode will be set to the last Language selected.
M. The Color Enhancer mode will be set to the last Color Enhancer mode
(NORMAL or WARM) selected.
The reset operation provides two functions for the CPU system First, when
power is first applied to the system the reset circuit will initiate a micro
computer program within the CPU. This sets the CPU into the conditions
described above. Second, at the time of a power failure and before the CPU
can go into disarray, the reset circuit clears any operating programs within the
CPU.
When a power failure occurs, the rest program keeps the TV in the Off
condition after the power is restored, until the power key is pressed.
Reset operation – AC power applied.
When AC is applied, the 5 VDC supply line to CPU pins 14 and 22 and the
emitter of Q831 will gradually rise. At 2 µs after the power supply line reaches
4.5 V, the collector of Q831 will change from a Low to High level. At this point
the CPU will execute a Reset On/Off, resetting the program counter to 0
(zero), and restart the program from the beginning.
Reset operation – AC power Off.
When the AC power is lost, the 5 VDC supply line to CPU pins 14 and 22 and
the emitter of Q831 will gradually begin to drop. At 15 µs after the power
supply line reaches 4.5 V, the collector of Q831 will change from a High to
Low level. At this point the CPU will execute a Reset Off/On, interrupt the
program in progress, reset the program counter to 0, and enter a standby
state. When the supply line reaches 4 V, the CPU will cease to function.
