X98024 Datasheet, PDF (20/29 Pages) Intersil Corporation – 240MHz Triple Video Digitizer with Digital PLL

X98024
OFFSET DAC
RANGE
0x0C[0]
0
1
0
1
TABLE 5. OFFSET DAC RANGE AND OFFSET DAC ADJUSTMENT
10 BIT
OFFSET DAC
RESOLUTION
ABLC™
0x17[0]
USER OFFSET CONTROL
RESOLUTION USING REGISTERS
0x09 - 0x0B ONLY
(8 BIT OFFSET CONTROL)
USER OFFSET CONTROL
RESOLUTION USING REGISTERS
0x09 - 0x0B AND 0x0C[7:2]
(10 BIT OFFSET CONTROL)
0.25 ADC LSBs
0
1 ADC LSB
N/A
(0.68mV)
(ABLC on)
(digital offset control)
0.125 ADC LSBs
0
1 ADC LSB
N/A
(0.34mV)
(ABLC on)
(digital offset control)
0.25 ADC LSBs
(0.68mV)
1
(ABLC off)
1.0 ADC LSB
(analog offset control)
0.25 ADC LSB
(analog offset control)
0.125 ADC LSBs
(0.34mV)
1
(ABLC off)
0.5 ADC LSB
(analog offset control)
0.125 ADC LSB
(analog offset control)
Clock Generation
A Digital Phase Lock Loop (DPLL) is employed to generate
the pixel clock frequency. The HSYNC input and the external
XTAL provide a reference frequency to the PLL. The PLL
then generates the pixel clock frequency that is equal to the
incoming HSYNC frequency times the HTOTAL value
programmed into registers 0x0E and 0x0F.
The stability of the clock is very important and correlates
directly with the quality of the image. During each pixel time
transition, there is a small window where the signal is
slewing from the old pixel amplitude and settling to the new
pixel value. At higher frequencies, the pixel time transitions
at a faster rate, which makes the stable pixel time even
smaller. Any jitter in the pixel clock reduces the effective
stable pixel time and thus the sample window in which pixel
sampling can be made accurately.
Sampling Phase
The X98024 provides 64 low-jitter phase choices per pixel
period, allowing the firmware to precisely select the optimum
sampling point. The sampling phase register is 0x10.
HSYNC Slicer
To further minimize jitter, the HSYNC inputs are treated as
analog signals, and brought into a precision slicer block with
thresholds programmable in 400mV steps with 240mV of
hysteresis, and a subsequent digital glitch filter that ignores
any HSYNC transitions within 100ns of the initial transition.
This processing greatly increases the AFE’s rejection of
ringing and reflections on the HSYNC line and allows the
AFE to perform well even with pathological HSYNC signals.
Voltages given above and in the HSYNC Slicer register
description are with respect to a 3.3V sync signal at the
HSYNC_IN input pin. To achieve 5V compatibility, a 680Ω
series resistor should be placed between the HSYNC source
and the HSYNC_IN input pin. Relative to a 5V input, the
hysteresis will be 240mV*5V/3.3V = 360mV, and the slicer
step size will be 400mV*5V/3.3V = 600mV per step.
The best HSYNC slicer threshold is generally 800mV (001b)
when locking on the rising edge of an HSYNC signal, or 2.4V
(110b) when locking on the falling edge.
SOG Slicer
The SOG input has programmable threshold, 40mV of
hysteresis, and an optional low pass filter than can be used
to remove high frequency video spikes (generated by
overzealous video peaking in a DVD player, for example)
that can cause false SOG triggers. The SOG threshold sets
the comparator threshold relative to the sync tip (the bottom
of the SOG pulse). A good default SOG slicer threshold
setting is 0x16 (hysteresis and low pass filter enabled,
threshold lowered slightly to accommodate weak sync tips).
SYNC Status and Polarity Detection
The SYNC Status register (0x01) and the SYNC Polarity
register (0x02) continuously monitor all 6 sync inputs
(VSYNC_IN, HSYNC_IN, and SOG_IN for each of 2 channels)
and report their status. However, accurate sync activity
detection is always a challenge. Noise and repetitive video
patterns on the Green channel may look like SOG activity
when there actually is no SOG signal, while non-standard
SOG signals and trilevel sync signals may have amplitudes
below the default SOG slicer levels and not be easily
detected. As a consequence, not all of the activity detect bits
in the X980xx are correct under all conditions.
Table 6 shows how to use the SYNC Status register (0x01)
to identify the presence of and type of a sync source. The
firmware should go through the table in the order shown,
stopping at the first entry that matches the activity indicators
in the SYNC Status register.
Final validation of composite sync sources (SOG or
Composite sync on HSYNC) should be done by setting the
Input Configuration register (0x05) to the composite sync
source determined by this table, and confirming that the
CSYNC detect bit is set.
The accuracy of the Trilevel Sync detect bit can be increased
by multiple reads of the Trilevel Sync detect bit. See the
Trilevel Sync Detect section for more details.
For best SOG operation, the SOG low pass filter (register
0x04[4]) should always be enabled to reject the high
frequency peaking often seen on video signals.
20
FN8220.0
June 6, 2005