LM2467 datasheet(18/43 Pages) NSC

18 / 43 page

OSD Generator Operation

PAGE OPERATION

Figure 13 shows the block diagram of the OSD generator.

OSD screens are 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 thw 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 anmy character row end characters.

The horizontal and vertical start position can also be pro-

grammed through the I

2C compatible bus.

END-OF-LINE AND END-OF-SCREEN CODES

There are two special character addresses used in the page

RAM, 0x00 (End-Of-Screen) and 0x01 (End-Of-Line). The

first must be used to terminate a window and the second to

terminate a line. Thus, only 254 ROM characters are display-

able.

BLANK CHARACTER REQUIREMENT

Five of the 256 Character ROM should be reserved as blank.

ROM Addresses 0 and 1 are for the use of the End-Of-

Screen and End-Of-Line characters as mentioned above.

ROM addresses 32, 64 ,and 255 must be reserved for test

engineering purposes. All other ROM addresses are usable,

and any that are unused must be filled with at least a

duplicate character. Any other addresses except for those

listed above should not be left blank.

DISPLAYING AN OSD IMAGE

Consecutive lines of characters make up the displayed win-

dow. These characters are stored in the page RAM through

the I

2C compatible bus. Each line can contain any number of

characters up to the limit of the displayable line length (de-

pendent on the pixels per line register), although some re-

strictions concerning the enhanced features apply on char-

acter lines 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 254 including any End-of-Line and End-of-Screen

characters. The horizontal and vertical start position can also

be programmed through the I

2C compatible bus.

WINDOWS

Two separate windows can be opened, utilizing the data

stored in the page RAM. Each window has its own horizontal

and vertical start position, although the second window

should be horizontally spaced at least two character spaces

away from the first window, and should never overlap the

first window when both windows are on. The OSD window

must be placed within the active video.

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 14, 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.

20075727

FIGURE 13. OSD Generator Block Diagram

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