LM1276 National Semiconductor Corporation, LM1276 Datasheet - Page 24

• Manufacturer Part Number: LM1276
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 512 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

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OSD Generator Operation
OSD VIDEO TIMING
Please refer to the LM1247 datasheet for details.
CHARACTER CELL
Please refer to the LM1247 datasheet for details.
FOUR COLOR FONT AS TWO 2-COLOR
Please refer to the LM1247 datasheet for details.
ATTRIBUTE TABLES
Each character has an attribute value assigned to it in the
page RAM. The attribute value is 4 bits wide, making each
character entry in the page RAM 13 bits wide in total. The
attribute value acts as an address, which points to one of 16
entries in either the two-color attribute table RAM or the
four-color attribute table RAM. The attribute word in the table
contains the coding information which defines which color is
represented by color 0 and color 1 in the two color attribute
table and color 0, color 1, color 2, color 3 in the four-color
attribute table. Each color is defined by a 9-bit value, with 3
bits assigned to each channel of RGB. A dynamic look-up
table defines each of the 16 different color ‘palettes’. As the
look-up table can be dynamically coded by the microcontrol-
ler over the I
assigned to any one of 2
maximum of 64 different colors to be used within one page
using the 4-color characters, with up to 4 different colors
within any one character and 32 different colors using the
2-color characters, with 2 different colors within any one
character.
TRANSPARENT DISABLE
In addition to the 9 lines of video data, a tenth data line is
generated by the transparent disable bit. When this line is
activated, the black color code will be translated as ‘trans-
parent’ or invisible. This allows the video information from
the PC system to be visible on the screen when this is
present. Note that this feature is enabled on any black color
in any of the first 8 attribute table entries.
VARIABLE TONE TRANSPARENCY
When the transparency is already in effect the tone of the
transparency can be adjusted. The contrast of the PC video
that is visible in the “transparent area” can be varied from
100% (fully transparent) to 0% (completely black). For ex-
ample, 50% reduction in contrast would provide a semi-
transparent effect. Just as in the conventional transparency
mode, variable tone transparency is effective on back-
grounds or foregrounds with black color codes from only the
first 8 attribute table entries. This feature is controlled by
0x85C0[6:0], and is only available when the transparency
mode is already enabled.
OSD WINDOW FADE IN/FADE OUT
The OSD window can be opened and closed with a fade
in/fade out effect. The interval for fading in and fading out the
OSD window in the horizontal and vertical direction is vari-
able and can be set by the microcontroller. This allows the
OSD window to be opened or closed in the vertical direc-
tions, horizontal direction, or from the upper left to lower right
corner. Assuming the desired time to typically complete a full
fade in or fade out is 0.5 seconds, and if the vertical scan
frequency is for example, 60 Hz, the number of steps is:
2
C compatible interface, each color can be
9
(i.e. 512) choices. This allows a
(Continued)
24
With a typical OSD window that is 300 pixels wide and 180
video lines long, the horizontal and vertical intervals would
be:
For a smooth fade in or fade out animation from the upper
left corner to the lower right corner, the horizontal to vertical
interval ratio must be matched to the aspect ratio of the OSD
window. In the example above, the 300 pixel wide by 180
lines long OSD window has an aspect ratio of 5:3. Thus, the
horizontal to vertical interval ratio should be set to 5/3 or
10/6. With an OSD window aspect ratio of 3:2, the H/V
intervals can be set to 3/2, 6/4, 9/6, 12/8, or 15/10 for optimal
operation. If the calculated aspect ratio of an OSD window is
a non-integer ratio, the H/V interval ratio should meet or
exceed the aspect ratio. For example, if the OSD aspect
ratio is 3.7:2 (or 1.85:1), the H/V intervals should be set to
2/1, 4/2, 6/3, 8/4, 10/5, or 12/6. The fade in/out speed
increases as H/V interval settings are increased. The OSD
window can also be faded in or out in only one direction if
desired, by setting the horizontal interval to 0 for fading in/out
strictly in the vertical direction or setting the vertical interval
to 0 for fading in/out in the horizontal direction. In interlaced
video formats, it is not recommended to fade in and fade out
the OSD in the vertical direction, and should be only faded in
the horizontal direction. The fade in/out function can be
enabled/disabled with bit 5 of the frame control register,
0x8400, and the horizontal & vertical intervals are controlled
by setting register 0x8429. The OSD window fade in/out
feature can only be used with OSD window 1.
ENHANCED FEATURES
Please refer to the LM1247 datasheet for details.
Microcontroller Interface
The microcontroller interfaces to the LM1276 preamp using
the I
begins with a Start Pulse followed by a byte comprised of a
7-bit Slave Device Address and a Read/Write bit. Since the
first byte is composed of both the address and the read/write
bit, the address of the LM1276 for writing is 0xBA
(10111010b) and the address for reading is 0xBB
(10111011b). The development software provided by Na-
tional Semiconductor will automatically take care of the dif-
ference between the read and write addresses if the target
address under the communications tab is set to 0xBA. Fig-
ures 17, 18 show a write and read sequence on the I
compatible interface.
WRITE SEQUENCE
The write sequence begins with a start condition, which
consists of the master pulling SDA low while SCL is held
high. The Slave Device Write Address, 0xBA, is sent next.
Each byte that is sent is followed by an acknowledge bit.
When SCL is high, the master will release the SDA line. The
slave must pull SDA low to acknowledge. The register to be
2
C compatible interface. The protocol of the interface
2
C