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338 AMD Geode™ LX Processors Data Book
Display Controller Register Descriptions
33234H
6.6.8 Video Downscaling
6.6.8.1 DC Video Downscaling Delta (DC_VID_DS_DELTA)
This register is provided to allow downscaling of the video overlay image by selective skipping of source lines. A DDA
engine is used to identify lines to be skipped according to the following algorithm:
At vertical retrace:
PHASE = 0; // clear PHASE initially
skip_flag = 0; // never skip the first line
linenum = 0; // point to first line
For each line of video: send_video_line(linenum); // send line to DF
linenum++ // increment to next line
{skip_flag, PHASE} = PHASE + DELTA; // skip_flag is carry from add
if (skip_flag) linenum = linenum + 1 // skip an additional line if flag was set
else linenum = linenum // otherwise, just skip n lines
The value to program into DC_VID_DS_DELTA is calculated as follows:
parms: DWORD ORIGINAL_LINES = full size image line count
DWORD SCALED_LINES = line count of scaled image equation:
DWORD DC_VID_DS_DELTA = ((ORIGINAL_LINES << 14) / SCALED_LINES) << 18;
Note: The scaling algorithm is only intended to work for ratios from 1 down to 1/2. The equation above clips the value to
the 14 bits of accuracy in the hardware. The equation could be modified to allow for higher bits in the future by
changing the 14-bit and 18-bit shift values. The only requirement is that the sum of the shift values be 32.
DC Memory Offset 080h
Typ e R /W
Reset Value 00000000h
DC_VID_DS_DELTA Register Map
313029282726252423222120191817161514131211109876543210
DELTA RSVD
VSYNC_SHIFT_EN
RSVD VSYNC_SHIFT
DC_VID_DS_DELTA Bit Descriptions
Bit Name Description
31:18 DELTA Delta. A 0.14 fixed-point fraction used as the delta value for the DDA engine that calcu-
lates which video lines to skip for video downscaling. This register is enabled when the
VDSE bit in DC_GENERAL_CFG is set (DC Memory Offset 004h[19] = 1).
17:16 RSVD Reserved.
15 VSYNC_SHIFT_
EN
VSYNC Shift Enable. When this bit is set, the VSYNC output is delayed during even
fields in interlaced modes. The amount of delay is defined in VSYNC_SHIFT (bits [11:0]).
14:12 RSVD Reserved.
11:0 VSYNC_SHIFT
VSYNC Shift. When VSYNC_SHIFT_EN is set (bit 15 = 1), this field determines the
number of dot clocks of delay that is inserted on VSYNC during even fields in interlaced
modes.