LM1238 datasheet(14/24 Pages) NSC

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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

range.

OSD MODES

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

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.

OSD SIZE AND POSITION

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.

20038761

FIGURE 12. Basic OSD Frame

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