LM1236 National Semiconductor Corporation, LM1236 Datasheet - Page 27

• 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

Register
FRMCTRL1
FRMCTRL2
CHARFONTACC
VBLANKDUR
CHARHTCTRL
BBHLCTRLB0
LM1236 Address Map
are used, one for 2-color characters, the other for 4-color characters. Each of the characters are stored in sequence in the Page
RAM. Special codes are used between lines to show where oneline ends and the next begins, and also to allow blank (or
’skipped’) single scan lines to be added between character rows.
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 below 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.
Bits 8–0 are all zeros. Bits 12–9 will have the previously entered AC but this is not used and so these bits are “don’t cares”.
OSD CONTROL REGISTERS
These registers, shown in Table 11, control the size, position, and enhanced features of up to two independent OSD windows. Any
bits marked as “X” are reserved and should be written to with zeros and should be ignored when the register is read. Additional
register detail is provided in the Control Register Definitions Section, later in this document.
ATTRIBUTE CODE
ATTRIBUTE CODE
ATTRIBUTE CODE
ATT[3:0]
ATT[3:0]
ATT[3:0]
Address
0x8400
0x8401
0x8402
0x8403
0x8404
0x8405
Default
0xFF
0x98
0x80
0x00
0x10
0x51
(Continued)
TABLE 11. OSD Control Register Detail
HTD
D7
PIXELS_PER_LINE[2:0]
X
X
X
0
0
TABLE 10. End-Of-Screen Code
TABLE 9. Skipped-Line Code
B[1:0]
TABLE 8. End-Of-Line Code
X
D6
0
0
X
X
27
0
X
0
FEN
D5
X
NUMBER OF SKIPPED LINES
END-OF-SCREEN CODE
CHAR_HEIGHT[7:0]
END-OF-LINE CODE
X
0
0
G[2:0]
VBLANK_DURATION[6:0]
D4
TD
X
X
0
0
CDPR
D3
X
BLINK_PERIOD[4:0]
0
0
D2E
D2
X
0
0
SL[3:0]
R[2:0]
ATTR
D1E
D1
0
0
www.national.com
FONT4
OSE
D0
1
0