LM1246DKBNANOPB National Semiconductor, LM1246DKBNANOPB Datasheet - Page 17

• Manufacturer Part Number: LM1246DKBNANOPB
• Manufacturer: National Semiconductor

Specifications of LM1246DKBNANOPB

• Lead Free Status / Rohs Status: Compliant

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 4 OSD pixels up to a
maximum of 16 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.
(0x843E[3])
(0x843A[2]) must be enabled. The number of steps is pro-
grammed with the bits in 0x843A[6:3]. When this feature is
disabled, please refer to the H-Blank Time Delay - On speci-
fication (+ Zero Crossing of I
end) listed under the System Interface Signal Characteristic
section.
Video Detection for Auto-Sizing &
Auto-Centering
The LM1246 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
FIGURE 12. Programmable Internal H. Blank
To
and
use
Programmable
this
feature,
HFB
to 50% of output blanking
Horizontal
Horizontal
20068552
Blanking
Blanking
17
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
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 LM1246 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 OSD
pixels per line mode when measurements are made. The
durations to be measured are shown generically in Figure 13
below, 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.
input or the horizontal flyback, in either horizontal lines
(vertical) or OSD pixels (horizontal).
edge of the sync or flyback pulse and the leading edge
of the first detected video, in either lines (vertical) or
OSD pixels (horizontal).
the last detected video and the leading edge of the sync
or flyback pulse, in either lines (vertical) or OSD pixels
(horizontal).
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