402-00005-00
Theory of Operation
2–32
Rev 02; February 8, 2002
HBLANK
Video
LEN
memory write enable
(valid video window)
H Offset
H Active
HOFF
HACT
Figure 2–31. PWG Horizontal Timing, Variable Scan Mode
In Figure 2–31, HACT and HOFF are programmed to crop the active video region. The selected edge of LEN starts
the horizontal counter; the positive or rising edge is shown in Figure 2–31. LEN starts the horizontal counter earlier,
requiring HOFF to be increased (notice the lack of front porch and sync pulse). HACT is also adjusted to compensate
for variable scan timing. The region of active video that falls within the HACT time is loaded into the memory. The
remainder of the video is cropped.
2.7.5.4 PWG Vertical Timing
Two values determine the number of lines loaded into the FIFO per frame. VACT holds the number of lines per frame
(including blanking). The selected edge of the incoming Vsync or FEN starts the vertical counter. The counter stops
when it reaches the value programmed in VACT, and waits for the next Vsync or FEN input. The value programmed
in VOFF determines the length of the vertical offset period (no line load region) with respect to the vertical count.
When the value in the VOFF register is reached the memory begins loading with active lines of pixel data until the
counter reaches the value programmed in VACT at which time loading stops. The two registers used together allow
cropping the video in the vertical direction.
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