Page
3.6
General Theory of Operation
• Horizontal Displayed Register
(R1)-
This 8-bit, write-only register
selects the number of characters
in
each row that will
be
displayed
on the screen.
In
normal operation, this register should contain the
value 50H or 80 decimal.
• Horizontal
Sync
Position
Register
(R2)-This
8-bit, write-only regis-
ter determines the position of the sync pulse
in
each scan line. Any
value greater than the value stored in the horizontal displayed register
(R1) and less than the value stored
in
the horizontal total register
(RO)
may be used. When the value programmed
in
R2
is
increased,
the display shifts to the left. When the value is decreased, the display
shifts to the right. The default value is 52H or
82
decimal.
•
Sync
Width
Register
(R3)-
This 8-bit, write-only register sets the
pulse width of the sync pulse. The horizontal sync
is
controlled by
the lower 4 bits, and the pulse width of the vertical sync by the upper
4 bits. Increasing this value lengthens the pulse. The default value
is
OFH
or 15 decimal.
NOTE: The sum of
R1
+
R2
+
R3
must
be
less than or equal
to
the
value
in
RO.
• Vertical Total Register
(R4)-
This 7-bit, write-only register
and
R5
are used to determine the refresh rate of the video monitor
and
the
vertical retrace timing. This value
is
expressed
in
character rows (nor-
mally 14 scan lines). These values are based
on
a horizontal rate
of
18,432 scan lines per second and are determined by the formula:
value
= horizontal scan lines per second/refresh rate x 1/scan lines
per character.
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