LM1237 datasheet(15/42 Pages) NSC

15 / 42 page

OSD Generator Operation

PAGE OPERATION

Figure 12. shows the block diagram of the OSD generator.

Video information is created using any of the 256 predefined

characters stored in the mask programmed ROM. Each

character has a unique 8-bit code that is used as its address.

Consecutive rows of characters make up the displayed win-

dow. These characters can be stored in the page RAM,

written under I

2C compatible commands by the monitor mi-

crocontroller. Each row can contain any number of charac-

ters up to the limit of the displayable line length, although

some restrictions concerning the enhanced features apply

on character rows longer than 32 characters.

The number of characters across the width and height of the

page can be varied under I

2C compatible control, but the

total number of characters that can be stored and displayed

on the screen is limited to 512 including any character row

end characters. The horizontal and vertical start position can

also be programmed through the I

2C compatible bus.

OSD VIDEO DAC

The OSD DAC is controlled by the 9-bit (3x3 bits) OSD video

information coming from the pixel serializer look-up table.

The look-up table in the OSD palette is programmed to

select 4 color levels out of 8 linearly spaced levels per

channel. The OSD DAC is shown in Figure 13, where the

gain is programmable by the 2-bit OSD CONTRAST register,

in 4 stages to give the required OSD signal. The OSD DACs

use the reference voltage, V

REF, to bias the OSD outputs.

20023419

FIGURE 12. Block Diagram of the OSD Generator

LM1237 数据表(PDF) 15 Page - National Semiconductor (TI)