38
Video Input/Output Daughter Card
UG235 (v1.2.1) October 31, 2007
Chapter 4:
DVI/VGA Input Interface
R
The VSOUT output is an unmodified signal from the selected source. In analog mode with
separate syncs, it is simply a passthrough from the VSYNC input. If the analog input is
using a composite sync mode, VSOUT is the recovered vertical sync. In DVI mode, VSOUT
is the decoded VSYNC from the serial digital stream.
In analog modes, the HSOUT is a reconstructed version of the HSYNC input or horizontal
sync from the composite sync. Registers allow adjustment of the polarity and duration of
this signal. In DVI mode, HSOUT is the decoded HSYNC from the serial digital stream.
DE is only available in DVI mode. This signal qualifies the pixel data as active pixels. In
analog mode, this signal is high, and the pixels must be qualified elsewhere.
DVI Input
The Digital Visual Interface (DVI) is a digital replacement for analog VGA. It is simply a
digital version of the above described analog interface, with the same scan order and
timing. Instead of analog voltages for the video data, the data is serially encoded digital
values. This method for high-speed serial data is called transition minimized differential
signaling (TMDS). TMDS is a combination of the electrical signal specification and the
encoding scheme. The electrical signal specifications are similar to LVDS. The encoding
scheme results in 10-bit symbols for each 8-bit byte, thus the encoded bit rate is 10x the byte
rate. Each pixel is encoded as a 24-bit value, 8-bits for each color. Just like VGA, each color
is transferred separately, so each color has its own differential pair. This means that the
encoded bit rate is 10x the pixel rate. DVI also requires a separate clock reference signal,
increasing the number of differential pairs to 4. The 10-bit encoding also includes some
special symbols, allowing the sync signals to be included with the green signal (a digital
SOG).
Other than the digital encoding, there is one significant difference in interfacing to DVI.
The TMDS encoding also allows a data enable (DE) signal to be carried with the data. This
signal is very useful to digital systems, as it easily qualifies the data. In the analog VGA
scheme, the only way to know when the data is valid is to know specific timing
relationships with respect to the sync signals.
I2C Initialization Table (in Hex)
All I2C communications to the AD9887A is at address 0x9a/9B. See
through
Table 4-2:
Analog VGA60
Register Name
Register
Address
Register
Value
Description
Active Interface
0x12
0x81
Force selection of analog input
PLL Div MSB
0x01
0x41
PLL divider value. VGA60 has 1056 cycles per HSYNC
period. 1056 -1 = 0x41F
PLL Div LSB
0x02
0xF0
VCO/CPMP
0x03
0x8C
VCORNGE = 00, CURRENT = 011
Phase Adjust
0x04
0x80
Default phase = T/2
Clamp Placement
0x05
0x24
36 cycles after HSYNC
Clamp Duration
0x06
0x24
36 cycles in duration
HSOUT Pulse width
0x07
0x80
128 cycles in HSYNC
www.BDTIC.com/XILINX