Mitsubishi Electric 1786FD2 Service Manual page 68

MEA 1786FD2 CRT Monitor Service Guide
3.MCU and the peripheral circuit operation theory:
3-1.MCU function:
The MCU is MTV112, it is an 87C51 with PWM output controlled microcontroller, after
power on,the reset circuit output a "High" to "Low" signal(>40mS) and the 12MHz crystal
oscillated circuit working , the MCU begin to manages the following functions,
(1) To detect mode and output proper SC0, SC1 and SC2 to deflection circuit.
(2) To check if there is the same saved mode in the EEPROM and get the data to transfer
into DC voltages by PWM output and RC filter circuits to control the picture, color, contrast
and brightness.
(3) To check if there is the valid key be pressed and do the key function.
(4) To memorize mode timings and any adjustable parameters of the picture into EEPROM.
(5) To output data to OSD IC for making an "on screen display control" menu.
(6) The inner registers and PWM output of MCU can be controlled by the external PC
alignment program.
(7) To calibrate the size, position, and geometry of the picture by pressing i-key. It will be
placed right size and position.
3-2.How to detect mode timing:
Only when the mode timing input is stable, we can adjust the picture and check the
horizontal and vertical sync frequency by the OSD menu, and the mode timing input mean the
horizontal sync signal and the vertical sync signal.
(1) The vertical sync frequency measurement:
We use the base timer, it can generate a count during a fixed time, this fixed time is
12/12MHz and we call it "Time_base", so when the first vertical sync generated, we enable
the base timer,and the next vertical sync generated,we disable the base timer,and we only need
to calculate how many
Vertical sync frequency
= FV
= 1 / Vertical sync period
= 1 / [ Counts * (Time_base)]
==> Vertical sync frequency = 1000000 / Counts
(2) The horizontal sync frequency measurement:
We use the event counter for calculating how many counts are during a long fixed time,
because the vertical sync period is longer than the horizontal sync period, we can enable the
event counter when the first vertical sync generated and disable the event counter when the
next vertical sync generated, this time,we can get the horizontal sync counts during a vertical
sync period.
The formula is Horizontal sync frequency
= FH
= Horizontal sync counts / Vertical sync period
==> Horizontal sync frequency
= Horizontal sync Counts / Vertical sync period
counts are during a vertical sync period. The formula is
Circuit Operation Theory
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