LM1246 Datasheet, PDF (19/51 Pages) National Semiconductor (TI) – 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM, 512 Character RAM and 4 DACs

OSD Generator Operation (Continued)
PAGE OPERATION
Figure 14 shows the block diagram of the OSD generator.
OSD screens are created using any of the 512 predefined
characters stored in the mask programmed ROM. The
LM1246 offers two modes of operation. The full 9-bit char-
acter code definition mode allows the entire 512 ROM
character set to be displayed at once. There is also an
8-bit character code definition mode, which ensures compat-
ibility with the LM1247. In this mode, the LM1246 works
exactly as a LM1247, where only half of the 512 ROM
characters can be displayed at any one time. The two differ-
ent modes can be selected with bit 2 of register 0x8439,
while the default mode is the standard LM1247 mode.
Please see the PAGE OPERATION section of the LM1247
datasheet for a detailed explanation on the standard
LM1247 mode’s 8-bit character code definition and bank
select operation. The more flexible 9-bit character code op-
eration enables all 512 character addresses to be indepen-
dently accessed on one page, however this mode requires
more information to be transmitted to specify a full 9-bit
character code. The standard LM1247 mode with an 8-bit
character code requires minimal I2C transmission as well as
minimal ROM in the monitor’s microcontroller, however the
application is limited to the display of only 254 out of the 512
characters on any single OSD menu page.
OSD ROM CONFIGURATION
The OSD ROM is equivalent to two 256 character ROMs of
the type used in the LM1253A and LM1237. When the
standard LM1247 is the selected mode of operation, where
the bank select method is in effect, each can be considered
as a group of 3 banks, (192) two-color characters followed
by 1 bank (64) four-color characters. Physically, the com-
bined ROM is then 192x2 + 64x4 + 192x2 + 64x4. This is
shown in Figure 14.
BANK ADDRESSING
A pictorial view of this addressing method is shown in Figure
15. On the left side is a section of the Page RAM with four
different addresses in successive locations, which have
been chosen to demonstrate accessing 4 of the 8 ROM
banks using the Bank Select Registers. The first has 10b for
the two most significant bits, so the OSD generator looks in
B2AD[2:0], located in Bank Select Register B, for its ROM
bank address. SInce B2AD[2:0] contains 101b, the character
font is read from Bank 5. The complete font address is
composed of this bank address, plus the lower six bits of the
original byte in Page RAM, giving a ROM address of
101101110b. The remaining addresses demonstrate that the
four selected banks can be displayed in any order.
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. The LM1246 is different from the LM1253A
and LM1237 in that these are now not actually encoded into
ROM, but are instead detected by the logic as the OSD
image is read from page RAM. This means that the two
lowest locations in the bank which is currently selected by
Bank Select Register 0, 0x8427[2:0], cannot be displayed in
an OSD image. However, these two characters can be
masked in the ROM, and if this bank is selected by Bank
Select Registers 1, 2 or 3, then these two characters are
usable on screen.
BLANK CHARACTER REQUIREMENT
Five of the 512 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 511 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 I²C 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 I²C 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 I²C compatible bus.
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