LM1247 National Semiconductor, LM1247 Datasheet - Page 24

• 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

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LM1247 Address Map
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. Bits 11-8 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, 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.
ATTRIBUTE CODE
The attribute code to be used for subsequent characters is written just as a character code or control code, but is stored with each
of the characters which are subsequently written to the Page RAM until the write operation ends. The attribute code is identified
as the first byte sent in a write operation. This attribute code addresses one of 16 locations in the attribute table, and is shown
in Table 10. This code is used for all following characters until a new attribute code is written with a new write operation.
Bits 7-4 of this byte are reserved and should be written as zeros. Bits 3-0 are stored in bits 11:8 of the Page RAM, along with its
corresponding character codes and selects one of the 16 entries in the attribute table (described later).
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 11 tells the OSD generator that the character and attribute codes which follow must be placed on a new line in the
displayed window. Bits 7-1 are zeros, bit 0 is a one. The attribute which 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 12, with the attribute bits setting the color of the skipped lines.
Bits 7-4 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 11-8 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 below for a case where skipped lines of zero characters are displayed, resulting
in one window being displayed in two segments.
ATTRIBUTE CODE
ATTRIBUTE CODE
ATTRIBUTE CODE
ATT[3:0]
ATT[3:0]
ATT[3:0]
X
(Continued)
X
TABLE 12. Skipped-Line Code
TABLE 11. End-of-Line Code
TABLE 9. Page RAM Format
X
TABLE 10. Attribute Code
BANK SEL.
X
0
CC[7:6]
ATTRIBUTE CODE
X
X
0
24
NUMBER OF SKIPPED LINES
X
0
END-OF-LINE CODE
CHARACTER CODE
ATT[3:0]
0
X
BANK CHARACTER
CC[5:0]
0
0
SL[3:0]
0
1