AD9880/PCB Analog Devices Inc, AD9880/PCB Datasheet - Page 16

• Manufacturer Part Number: AD9880/PCB
• Description: BOARD EVALUATION PCB AD9880
• Manufacturer: Analog Devices Inc

Specifications of AD9880/PCB

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD9880
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.
There is a pipeline in the AD9880, which must be flushed
before valid data becomes available. This means 23 data sets are
presented before valid data is available.
The timing diagram in Figure 7 shows the operation of the
AD9880.
Hsync Timing
Horizontal Sync (Hsync) is processed in the AD9880 to
eliminate ambiguity in the timing of the leading edge with
respect to the phase-delayed pixel clock and data.
The Hsync input is used as a reference to generate the pixel
sampling clock. The sampling phase can be adjusted, with
respect to Hsync, through a full 360° in 32 steps via the phase
adjust register (to optimize the pixel sampling time). Display
systems use Hsync to align memory and display write cycles, so
it is important to have a stable timing relationship between the
Hsync output (HSOUT) and data clock (DATACK).
Three things happen to Hsync in the AD9880. First, the polarity
of Hsync input is determined and thus has a known output
polarity. The known output polarity can be programmed either
active high or active low (Register 0x24, Bit 7). Second, HSOUT
is aligned with DATACK and data outputs. Third, the duration
of HSOUT (in pixel clocks) is set via Register 0x23. HSOUT is
the sync signal that should be used to drive the rest of the
display system.
Coast Timing
In most computer systems, the Hsync signal is provided
continuously on a dedicated wire. In these systems, the Coast
input and function are unnecessary, and should not be used and
the pin should be permanently connected to the inactive state.
In some systems, however, Hsync is disturbed during the verti-
cal sync period (Vsync). In some cases, Hsync pulses disappear.
In other systems, such as those that employ composite sync
(Csync) signals or embedded SOG, Hsync includes equalization
pulses or other distortions during Vsync. To avoid upsetting the
DATACK
HSOUT
DATA
t
DCYCLE
t
SKEW
Figure 7. Output Timing
t
PER
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clock generator during Vsync, it is important to ignore these
distortions. If the pixel clock PLL sees extraneous pulses, it
attempts to lock to this new frequency, and changes frequency
by the end of the Vsync period. It then takes a few lines of
correct Hsync timing to recover at the beginning of a new
frame, resulting in a tearing of the image at the top of the
display.
The Coast input is provided to eliminate this problem. It is an
asynchronous input that disables the PLL input and allows the
clock to free run at its then-current frequency. The PLL can free
run for several lines without significant frequency drift.
Coast can be generated internally by the AD9880 (see
Register 0x12 [1]), can be driven directly from a Vsync input,
or can be provided externally by the graphics controller.
Sync Processing
The inputs of the sync processing section of the AD9880 are
combinations of digital Hsyncs and Vsyncs, analog sync-on-
green, or sync-on-Y signals, and an optional external Coast
signal. From these signals it generates a precise, jitter-free (9%
or less at 95 MHz) clock from its PLL; an odd-/even-field signal;
Hsync and Vsync out signals; a count of Hsyncs per Vsync; and
a programmable SOG output. The main sync processing blocks
are the sync slicer, sync separator, Hsync filter, Hsync regen-
erator, Vsync filter, and Coast generator.
The sync slicer extracts the sync signal from the green graphics
or luminance video signal that is connected to the SOGIN input
and outputs a digital composite sync. The sync separator’s task
is to extract Vsync from the composite sync signal, which can
come from either the sync slicer or the Hsync input. 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 Vsync filter is used to elimi-
nate spurious Vsyncs, maintain a stable timing relationship
between the Vsync and Hsync output signals, and generate the
odd/even field output. The Coast generator creates a robust
Coast signal that allows the PLL to maintain its frequency in
the absence of Hsync pulses.