LM1253A National Semiconductor, LM1253A Datasheet - Page 23

• Manufacturer Part Number: LM1253A
• Description: Monolithic Triple 180 MHz I2C CRT Pre-amp With Integrated Analog On Screen Display (OSD) Generator
• Manufacturer: National Semiconductor

OSD Generator Operation
Page Operation
The block diagram of the OSD generator is shown Figure 32 .
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FIGURE 32. Block Diagram of the OSD Generator
Video information is created using any of the 256 pre-
defined characters stored in the mask programmed ROM.
Each character has a unique 8 bit code that is used as its ad-
dress. Consecutive rows of characters make up the dis-
played window. These characters can be stored in the page
2
RAM, written under I
C controlled commands by the monitor
micro-controller. Each row can contain any number of char-
acters 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
2
page can be varied under I
C control, but the total number of
characters that can be stored and displayed on the screen is
limited to 512 including any row end, skip line, and window
end control characters. The horizontal and vertical start posi-
2
tion can also be programmed under I
C control.
OSD Video DAC
The OSD DAC is controlled by the 9 bit (3x9 bits) OSD video
information coming from the pixel serializer look-up table.
The OSD DAC is shown in the Figure 33 , where the gain is
programmable by the 2 bit OSD CONTRAST register, in 4
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stages to give the required OSD signal.
FIGURE 33. Block Diagram of OSD DACs
The analog signal created by the OSD DACs is biased with
respect to V
.
REF
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