LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 26

• Manufacturer Part Number: LM1236
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 254 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

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LM1236 Address Map
DISPLAY PAGE RAM
This address range (0x8000–0x81FF) contains the 512 characters, which comprise the displayable OSD screens. There must be
at least one End-Of-Screen code (0x00) in this range to prevent unpredictable behavior. NOTE: To avoid any unpredictable
behavior, this range should be cleared by writing a 0 to bit 3 of the FRMCTRL1 Register, 0x8400, immediately after power up.
There may also be one or more pairs of End-Of-Line and Skip Line codes. The codes and characters are written as 8-bit bytes,
but are stored with their attributes in groups of 13 bits. When writing, one byte describes a displayed character (CC), Attribute
Code (AC), End-Of-Screen (EOS), End-Of-Line (EOL) or Skip Line (SL) code.
When reading characters from RAM, bit 1 of the Character Font Access Register (0x8402) determines whether the lower 8 bits
or upper 4 bits of the Page RAM are returned. Table 5 gives the lower byte read, which is the first 8 character code bits when bit
1 of the Character Font Access Register is a 0. Table 6 gives the upper byte read, which is 4 attribute code bits when this bit is
set to a 1.
RAM Data Format
Each of the 512 locations in the page RAM is comprised of a 12-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 7 shows the format of a character stored in RAM. Note that even though this is a 12- bit format,
reading and writing characters and codes is done in 8-bit bytes.
Bits 11–4 (Character Code): These 8 bits define which of the 254 characters is to be called from the character ROM. Valid
character codes are 0x02-0xFF. Bits 3–0 (Attribute Code): these 4 bits address the attribute table used to specify which of the 16
locations in RAM specify the colors and enhanced features to be used for this particular character. Two seperate attribute tables
Address Range
0x0000–0x2FFF
0x3000–0x3FFF
0x4000–0x7FFF
ATTRIBUTE CODE
Address Range
0x8000–0x81FF
0x8000–0x81FF
Address
ATT[3:0]
R/W
(Continued)
R
R
-
TABLE 5. Page RAM Lower Byte Read Data
TABLE 6. Page RAM Upper Byte Read Data
TABLE 4. Character ROM Addressing
D7
D7
X
TABLE 7. Page RAM Format
ROM Character Fonts, 190 two-color Character Fonts that are
read-only. The format of the address is as follows: A15-A14: Always
zero. A13-A16: Character value (0x00-0xBF are valid values)
A5-A1: Row of the character (0x00-0x011 are valid values) A0: Low
byte of line when a zero. High byte of line when a one. The low
byte will contain the first eight pixels of the line with data Bit 0
corresponding to the left most bit in the Character font line. The
high byte will contain the last four pixels and data. Bits 7-4 are
"don’t cares". Data Bit 3 of the high byte corresponds to the right
most pixel in the Character Font line.
ROM Character Fonts, 64 four-color Character Fonts that are
read-only. The format of the address is as follows: A15-A14: Always
zero. A13-A16: Character value (0xC0-0xFF are valid values)
A5-A1: Row of the character (0x00-0x011 are valid values) A0: Low
byte of line when a zero. High byte of line when a one. The low
byte will contain the first eight pixels of the line with data Bit 0
corresponding to the left most bit in the Character font line. The
high byte will contain the last four pixels and data. Bits 7-4 are
"don’t cares". Data Bit 3 of the high byte corresponds to the right
most pixel in the Character Font line.
Reserved
D6
D6
X
26
D5
D5
X
CHARACTER CODE
CHAR_CODE[7:0]
D4
D4
X
CC[7:0]
Description
D3
D3
ATTR_CODE[3:0]
D2
D2
D1
D1
D0
D0