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LM1238 Datasheet(PDF) 14 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM1238
Description  110 MHz I2C Compatible RGB Preamplifier with Internal Simple OSD Generator and 4 DACs
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com

LM1238 Datasheet(HTML) 14 Page - National Semiconductor (TI)

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OSD Operation
The LM1238 is designed to enable ultra-low cost OSD op-
eration in monitors that usually use a simple button + LED
interface instead of OSD.
The basic screen is formed by a frame as shown in figure 12
below. The frame consists of a screen image with a bar
graph beneath the image and an index pointer on the right
hand side. This basic frame is common to all different views
of the OSD. The blank ’screen image’ of the screen in figure
12 is used to display one (or one set) of the available icons
which would correspond to the adjustment being carried out.
The available icons are shown in Figures 13 and 14. The 16
position index pointer is used to provide an indication of
which icon set of the 16 available is currently displayed. The
16 icon sets can be arranged in any order. The bar graph is
adjustable to 32 different lengths and can be used to indicate
where the adjustment level is in the available adjustment
Two modes of operation are available: alternating sequence
and fixed icon. In the alternating sequence mode, the display
will sequence through the three icons in the selected icon
set. For example, if the color temperature icon set (Icon set
15) is selected, the icon display will sequence from the 9300
icon (Screen 0) to the 6500 icon (Screen 1) to the 5000 icon
(Screen 2), then back to the 9300 icon (Screen 0). This
sequence will be continuously repeated under hardware
control. The rate of the sequence is controllable in incre-
ments of 8 frames of video by programming the lower 4 bits
of control register 0x0804 via the I2C interface. The display
is also capable of continuously displaying one of the three
icons in the set by programming bits 4 and 5 of control
register 0x0804.
OSD Constant Character Height Function
A constant height function is included in the LM1238 to
maintain a constant OSD height as the number of horizontal
image lines varies. Certain OSD character lines must be
repeated in order to maintain a constant height as a percent-
age of the vertical image size. The constant character height
function implements an approximation algorithm that deter-
mines which lines are repeated, and how many times each
line is repeated. This will maintain a relatively constant
height and position for the OSD window.
The size and position of the OSD is controlled by two regis-
ters. The available selections for horizontal size are 130,
176, 240, and 352 pixels per line. The horizontal position is
controllable in 64 steps of 8 pixels. The start point is refer-
enced to the horizontal blank pulse. The available selections
for the vertical height of the OSD are 95, 130, 180, and 240
lines per field. The vertical position is adjustable in 64 incre-
ments of 8 OSD lines. The start point is referenced to the
vertical blank pulse.
FIGURE 12. Basic OSD Frame

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