AD9398
Rev. 0 | Page 10 of 44
TIMING
The output data clock signal is created so that its rising edge
always occurs between data transitions and can be used to latch
the output data externally.
Figure 3 shows the timing operation of the AD9398.
t
PER
t
DCYCLE
t
SKEW
DATAC
05678-003
DATA
HSOUT
Figure 3. Output Timing
VSYNC FILTER AND ODD/EVEN FIELDS
The VSYNC filter is used to eliminate spurious VSYNCs, maintain
a consistent timing relationship between the VSYNC and
HSYNC output signals, and generate the odd/even field output.
The filter works by examining the placement of VSYNC
with respect to HSYNC and, if necessary, slightly shifting
it in time at the VSOUT output. The goal is to keep the
VSYNC and HSYNC leading edges from switching at the
same time, eliminating confusion as to when the first line
of a frame occurs. Enabling the VSYNC filter is done with
Register 0x21[5]. Use of the VSYNC filter is recommended for
all cases, including interlaced video, and is required when using
the HSYNC per VSYNC counter.
Figure 4 and Figure 5
illustrate even/odd field determination in two situations.
FIELD 1 FIELD 0
SYNC SEPARATOR THRESHOLD
FIELD 1 FIELD 0
23 21431
HSIN
VSIN
4
VSOUT
O/E FIELD
EVEN FIELD
QUADRAN
05678-004
Figure 4. VSYNC Filter—Even
FIELD 1 FIELD 0
SYNC SEPARATOR THRESHOLD
FIELD 1 FIELD 0
23 214431
HSIN
VSIN
VSOUT
O/E FIELD
ODD FIELD
QUADRAN
05678-005
Figure 5. VSYNC Filter—Odd
HDMI RECEIVER
The HDMI receiver section of the AD9398 allows the reception
of a digital video stream, which is backward compatible with
DVI and able to accommodate not only video of various for-
mats (RGB, YCrCb 4:4:4, 4:2:2), but also up to eight channels of
audio. Infoframes are transmitted carrying information about
the video format, audio clocks, and many other items necessary
for a monitor to use fully the information stream available.
The earlier digital visual interface (DVI) format was restricted
to an RGB 24-bit color space only. Embedded in this data
stream were HSYNCs, VSYNCs, and display enable (DE)
signals, but no audio information. The HDMI specification
allows transmission of all the DVI capabilities, but adds several
YCrCb formats that make the inclusion of a programmable
color space converter (CSC) a very desirable feature. With this,
the scaler following the AD9398 can specify that it always
wishes to receive a particular format—for instance, 4:2:2 YCrCb—
regardless of the transmitted mode. If RGB is sent, the CSC can
easily convert that to 4:2:2 YCrCb while relieving the scaler of
this task.
In addition, the HDMI specification supports the transmission
of up to eight channels of S/PDIF or I
2
S audio. The audio
information is packetized and transmitted during the video
blanking periods along with specific information about the
clock frequency. Part of this audio information (audio
infoframe) tells the user how many channels of audio are being
transmitted, where they should be placed, information
regarding the source (make, model), and other data.
DE GENERATOR
The AD9398 has an on-board generator for DE, for start of
active video (SAV), and for end of active video (EAV), all of
which are necessary for describing the complete data stream for
a BT656-compatible output. In addition to this particular
output, it is possible to generate the DE for cases in which a
scaler is not used. This signal alerts the following circuitry as to
which are displayable video pixels.
