LM1247 National Semiconductor, LM1247 Datasheet - Page 15

• Manufacturer Part Number: LM1247
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM, 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

OSD Generator Operation
PAGE OPERATION
Figure 12 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 OSD
image is composed of up to 512 characters stored in page
RAM, where each character has a unique 8-bit address. This
means only half of the 512 ROM characters can be dis-
played at any one time. Since a 9 bit address is needed to
select from 512 ROM addresses, a bank select register is
used to convert the upper 2 bits of the character code into a
3 bit bank address which is combined with the other 6 bits to
produce the necessary 9 bits. Also, since the lower 6 bits can
address 64 characters, this gives a bank size of 64. There-
fore, the ROM consists of 8 banks of 64 characters each,
where 4 of these 8 banks are displayed by loading the 4
bank address registers with the appropriate 3 bit code. This
allows the programmer to switch between two different OSD
images, by simply reprogramming the bank addresses.
OSD ROM CONFIGURATION
The OSD ROM is equivalent to two 256 character ROMs of
the type used in the LM1253A and LM1237. Because of the
bank select method described earlier, each can be consid-
ered as a group of 3 banks (192) two-color characters fol-
lowed by 1 bank (64) four-color characters. Physically, the
combined ROM is then 192x2 + 64x4 + 192x2 + 64x4. This
is shown in Figure 12.
BANK ADDRESSING
A pictorial view of this addressing method is shown in Figure
13. 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
FIGURE 12. OSD Generator Block Diagram
15
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 LM1247 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. The consequences of this is that only 254
ROM characters are displayable at one time.
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
2
C compatible bus. Each line can contain any number of
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