LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 18

• Manufacturer Part Number: LM1236
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 254 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

www.national.com
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
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
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
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.
2
2
C compatible control, but the total number of
C compatible commands by the monitor mi-
2
C compatible bus.
FIGURE 13. OSD Generator Block Diagram
18
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
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
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
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
2
C compatible bus. Each line can contain any number of
2
C compatible control, but the total number of
REF
, to bias the OSD outputs.
2
C compatible bus.
20075727