AD9880
Rev. 0 | Page 18 of 64
Sync Separator
As part of sync processing, the sync separator’s task is to extract
Vsync from the composite sync signal. It works on the idea that
the Vsync signal stays active for a much longer time than the
Hsync signal. By using a digital low-pass filter and a digital
comparator, it rejects pulses with small durations (such as
Hsyncs and equalization pulses) and only passes pulses with
large durations, such as Vsync (see Figure 9).
The threshold of the digital comparator is programmable for
maximum flexibility. To program the threshold duration, write
a value (N) to Register 0x11. The resulting pulse width is
N × 200 ns. So, if N = 5 the digital comparator threshold is 1 µs.
Any pulses less than 1 µs is rejected, while any pulse greater
than 1 µs passes through.
The sync separator on the AD9880 is simply an 8-bit digital
counter with a 6 MHz clock. It works independently of the
polarity of the composite sync signal. Polarities are determined
elsewhere on the chip. The basic idea is that the counter counts
up when Hsync pulses are present. But since Hsync pulses are
relatively short in width, the counter only reaches a value of N
before the pulse ends. It then starts counting down until
eventually reaching 0 before the next Hsync pulse arrives. The
specific value of N varies for different video modes, but is
always less than 255. For example with a 1 s width Hsync, the
counter only reaches 5 (1 s/200 ns = 5). Now, when Vsync is
present on the composite sync the counter also counts up.
However, since the Vsync signal is much longer, it counts to a
higher number, M. For most video modes, M is at least 255.
So, Vsync can be detected on the composite sync signal by
detecting when the counter counts to higher than N. The
specific count that triggers detection, T, can be programmed
through the Serial Register 0x11.
Once Vsync has been detected, there is a similar process to
detect when it goes inactive. At detection, the counter first
resets to 0, then starts counting up when Vsync finishes.
Similarly to the previous case, it detects the absence of Vsync
when the counter reaches the threshold count, T. In this way, it
rejects noise and/or serration pulses. Once Vsync is detected to
be absent, the counter resets to 0 and begins the cycle again.
There are two things to keep in mind when using the sync
separator. First, the resulting clean Vsync output is delayed
from the original Vsync by a duration equal to the digital
comparator threshold (N × 200 ns). Second, there is some
variability to the 200 ns multiplier value. The maximum varia-
bility over all operating conditions is ±20% (160 ns to 240 ns).
Since normal Vsync and Hsync pulse widths differ by a factor of
about 500 or more, 20% variability is not an issue.
Hsync Filter and Regenerator
The Hsync filter is used to eliminate any extraneous pulses from
the Hsync or SOGIN inputs, outputting a clean, low-jitter signal
that is appropriate for mode detection and clock generation.
The Hsync regenerator is used to recreate a clean, although not
low jitter, Hsync signal that can be used for mode detection and
counting Hsyncs per Vsync. The Hsync regenerator has a high
degree of tolerance to extraneous and missing pulses on the
Hsync input, but is not appropriate for use by the PLL in
creating the pixel clock because of jitter.
The Hsync regenerator runs automatically and requires no
setup to operate. The Hsync filter requires the setting up of a
filter window. The filter window sets a periodic window of time
around the regenerated Hsync leading edge where valid Hsyncs
are allowed to occur. The general idea is that extraneous pulses
on the sync input occur outside of this filter window and thus
are filtered out. To set the filter window timing, program a value
(x) into Register 0x20. The resulting filter window time is ±x
times 25 ns around the regenerated Hsync leading edge. Just
as for the sync separator threshold multiplier, allow a ±20%
variance in the 25 ns multiplier to account for all operating
conditions (20 ns to 30 ns range).
A second output from the Hsync filter is a status bit (Reg-
ister 0x16[0]) that tells whether extraneous pulses are present
on the incoming sync signal or not. Extraneous pulses are often
included for copy protection purposes; this status bit can be
used to detect that.
The filtered Hsync (rather than the raw Hsync/SOGIN signal)
for pixel clock generation by the PLL is controlled by
Register 0x21[6]. The regenerated Hsync (rather than the
raw Hsync/SOGIN signal) for sync processing is controlled by
Register 0x21[7]. Use of the filtered Hsync and regenerated
Hsync is recommended. See Figure 10 for an illustration of a
filtered Hsync.
