LM1247 datasheet(16/48 Pages) NSC

16 / 48 page

OSD Generator Operation (Continued)

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 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.

OSD VIDEO TIMING

The OSD analog signal then goes to the switch, shown in

Figure 14 and Figure 1 where the timing control switches

from input video to OSD and back again as determined by

the control registers. This is also where horizontal and ver-

tical blanking are also inserted at their appropriate intervals.

20048428

FIGURE 13. Bank Addressing

LM1247 Datasheet(PDF) 16 Page - National Semiconductor (TI)