LM1276 National Semiconductor Corporation, LM1276 Datasheet - Page 20

• 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

www.national.com
Burn In Screen
The LM1276 provides a Burn In Screen feature, where a full
screen of white video will be displayed without the need for
any video input. The Burn In Screen is enabled by setting bit
5 of register 0x8439 to 1. The contrast level of this Burn In
display can be adjusted with a 2 Bit DAC over the range of
65% to 85% of normal white level video. This adjustment is
made with register 0X8439[7:6].
Programmable Horizontal Blank
The leading edge position of the internal horizontal blank can
be programmed with respect to the horizontal flyback zero
crossing leading edge in steps of 1 OSD pixel up to a
maximum of 31 steps as shown below in Figure 12 . This
start position of the horizontal blanking pulse is only pro-
grammable to occur before the horizontal flyback zero cross-
ing edge, and cannot be programmed in the opposite direc-
tion. The trailing edge of the horizontal blanking pulse is
independent of the programmable leading edge, and its
relativity to the Horizontal flyback trailing edge remains un-
changed. To use this feature, both Horizontal Blanking
(0x843A[0])
(0x843A[2]) must be enabled. The number of steps is pro-
grammed with the bits in 0x843A[7:3]. When this feature is
disabled, please refer to the H-Blank Time Delay – On
specification (+ Zero Crossing of I
blanking end) listed under the System Interface Signal Char-
acteristics section.
Video Detection for Auto-Sizing &
Auto-Centering
The LM1276 is capable of taking measurements necessary
for the monitor’s microcontroller to perform the auto-sizing
and auto-centering operations. The horizontal and vertical
flybacks/syncs are used as the reference for timing. Either
the flyback or sync signals may be used. In this section, the
flyback signals will be considered, although horizontal and
vertical sync can be applied similarly. The resultant outputs
are the flyback time, the position of the start of video relative
to the flyback end, and the time from the end of the active
video to the start of the flyback time. Since the total line time
FIGURE 12. Programmable Internal H. Blank
and
Programmable
HFB
Horizontal
to 50% of output
20096952
Blanking
20
is known, the microcontroller can calculate the active video
time. The microcontroller can center the video between the
start and end of flyback for best image centering, and to
calculate the duty cycle of the video with respect to the
forward scan time, thus giving a measure of the relative size
of the image.
VIDEO INPUT DETECTION
The LM1276 will detect even low-level video information to
determine the video image size and position. The video
detect logic must also find the extreme points of the dis-
played image during each frame with respect to the horizon-
tal and vertical flyback pulses as measured using the internal
PLL. For best performance in the auto-sizing mode, it is
recommended that the application use the maximum pixels
per line mode is used when measurements are made. The
durations to be measured are shown generically in Figure 13
and apply to both horizontal and vertical timings. Since
measurements are made in terms of OSD pixels, the
ratio of OSD pixels per line to the video pixels per line
must be applied to the data below to attain measure-
ments in terms of video pixels.
1. Flyback or sync period: The duration of either the sync
2. Back porch period: The duration between the trailing
3. Front porch: The duration between the trailing edge of
The video period is the duration between the leading edge of
the first detected video and the trailing edge of the last
detected video, in either lines (vertical) or OSD pixels (hori-
zontal). This period is calculated by the microcontroller with
the measured periods above.
As the video may start and finish at different positions on the
screen, depending upon the image, the measured horizontal
porches and video time may vary from line to line. To over-
come this, the periods should be measured over at least one
entire field. The hardware records the shortest back porch
and front porch periods used over the measured period. The
possible error for the above measurements are within 1 to 2
OSD pixels.
The video period is the duration between the leading edge of
the first detected video and the trailing edge of the last
detected video, in either lines (vertical) or OSD pixels (hori-
zontal). This period is calculated by the microcontroller with
the measured periods above.
The analog front end video detection is also utilized by the
Video Data Interface for decoding of color bar data. It is
critical to note again that the threshold for video detec-
tion is 80mV above the internal V
the minimum rise time requirement of the driving PC
video card must be at least 10.0ns. Excessively slow rise
times in the PC video card will prevent the video detect
circuit from working properly.
To perform an auto size calculation, the following instruction
sequence must be done by the MCU:
input or the horizontal flyback, in either horizontal lines
(vertical) or pixels (horizontal).
edge of the sync or flyback pulse and the leading edge
of the first detected video, in either lines (vertical) or
pixels (horizontal).
the last detected video and the leading edge of the sync
or flyback pulse, in either lines (vertical) or pixels (hori-
zontal).
REF
voltage, and also