LM1276AAA National Semiconductor, LM1276AAA Datasheet - Page 27

• Manufacturer Part Number: LM1276AAA
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM, 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

LM1276 Address Map
RAM Data Format
Each of the 512 locations in the page RAM is comprised of a 13-bit code consisting of an 9-bit character or control code, and a
4-bit attribute code. Each of the characters is stored in sequence in the page RAM in bits 8:0. Special codes are used between
lines to show where one line ends and the next begins, and also to allow blank (or ‘skipped’) single scan lines to be added
between character lines. Table 7 shows the format of a character stored in RAM. Note that even though this is a 13-bit format,
reading and writing characters and codes is done in 8-bit bytes.
Bits 8-0 determined which of the 512 characters is to be called from the character ROM. Bits 12–9 address one of the 16
attributes in the table containing the colors and enhanced features to be used for this particular character. Two separate attribute
tables are used, one for 2-color characters, and the other for 4-color characters. Note there are 16 available attributes for 2-color
characters and a different set of 16 available attributes for 4-color characters.
End-Of-Line Code
To signify the end of a line of characters, a special End-Of-Line (EOL) code is used in place of a character code. This code, shown
in Table 8 tells the OSD generator that the character and attribute codes which follow must be placed on a new line in the
displayed window. Bits 8–1 are zeros, bit 0 is a one. The attribute that is stored in Page RAM along with this code is not used.
Skip-Line Code
In order to allow finer control of the vertical spacing of character lines, each displayed line of characters may have up to 15
skipped (i.e., blank) lines between it and the line beneath it. Each skipped line is treated as a single character pixel line, so
multiple scan lines may actually be displayed in order to maintain accurate size relative to the character cell. An internal algorithm
maintains vertical height proportionality (see the section on Constant Character Height Mechanism). To specify the number of
skipped lines, the first character in each new line of characters is interpreted differently than the others in the line. Its data are
interpreted as shown in Table 9, with the attribute bits setting the color of the skipped lines.
Bits 8–4 are reserved and should be set to zero. Bits 3–0 determine how many blank pixel lines will be inserted between the
present line of display characters and the next. A range of 0–15 may be selected. Bits 12–9 determine which attribute the pixels
in the skipped lines will have, which is always called from the two-color attribute table. The pixels will have the background color
(Color 0) of the selected attribute table entry.
Note that the pixels in the first line immediately below the character may be overwritten by the pixel override system that creates
the button box. (Refer to the Button Box Formation Section for more information).
After the first line, each new line always starts with an SL code, even if the number of skipped lines to follow is zero. This means
an SL code must always follow an EOL code. An EOL code may follow an SL code if several ‘transparent’ lines are required
between sections of the window. See Example 3 in the LM1247 data sheet for a case where skipped lines of zero characters are
displayed, resulting in one window being displayed in two segments.
End-Of-Screen Code
To signify the end of the window, a special End-Of-Screen (EOS) code is used in place of a End-Of-Line (EOL) code. There must
be at least one EOS code in the Page RAM to avoid unpredictable behavior. This can be accomplished by clearing the RAM by
writing a 0 to bit 3 of the FRMCTRL1 Register, 0x8400, immediately after power up.
ATTRIBUTE CODE
ATTRIBUTE CODE
ATTRIBUTE CODE
Address Range
0x8000–0x81FF
0x8000–0x81FF
ATT[3:0]
ATT[3:0]
Address
ATT[3:0]
(Continued)
TABLE 5. Page RAM Lower Byte Read Data
TABLE 6. Page RAM Upper Byte Read Data
TABLE 7. Page RAM Format (9-bit mode)
X
0
D7
D7
X
TABLE 9. Skipped-Line Code
TABLE 8. End-Of-Line Code
X
0
D6
D6
X
27
0
X
D5
D5
X
NUMBER OF SKIPPED LINES
END-OF-LINE CODE
X
0
CHARACTER CODE
CHAR_CODE[7:0]
CC[8]
D4
D4
CC[8:0]
X
0
D3
D3
0
ATTR_CODE[3:0]
D2
D2
0
SL[3:0]
D1
D1
0
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D0
D0
1