LM1246AAA National Semiconductor, LM1246AAA Datasheet - Page 28

• Manufacturer Part Number: LM1246AAA
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM/ 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

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LM1246 Address Map
RAM Data Format (Standard LM1247 Mode)
Each of the 512 locations in the page RAM is comprised of a 13-bit code consisting of an 8-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 7:0. 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 8. Page RAM Format (Standard Mode) 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 7–6 determine which Bank Select Register is used to look up the 3-bit address of the bank where the character will be called
from. Bits 5–0 determine which of the 64 characters is called from that bank. Bit 8 is unused since this would be the 9th character
code bit for the mode below, however in this mode, the character code is defined with only 8 bits. 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. It is the bank number in the register
called by the Bank Select bits, which determines whether the character has a 2-color or 4-color attribute.
RAM Data Format (9-Bit Character Code Definition Mode)
Each of the 512 locations in the page RAM is comprised of a 13-bit code consisting of a 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 9. Page RAM Format (9-bit mode) 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 10. End-Of-Line Code 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 11. Skipped-Line Code, 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.
ATTRIBUTE CODE
ATTRIBUTE CODE
ATTRIBUTE CODE
ATTRIBUTE CODE
ATT[3:0]
ATT[3:0]
ATT[3:0]
ATT[3:0]
(Continued)
TABLE 8. Page RAM Format (Standard Mode)
TABLE 9. Page RAM Format (9-bit mode)
X
0
CC[8]
TABLE 11. Skipped-Line Code
TABLE 10. End-Of-Line Code
0
X
0
BANK SEL.
CC[7:6]
28
X
0
NUMBER OF SKIPPED LINES
END-OF-LINE CODE
0
X
CHARACTER CODE
CHARACTER CODE
CC[8:0]
X
0
BANK CHARACTER
0
CC[5:0]
0
SL[3:0]
0
1