Fusion 878A
3.0 Electrical Interfaces
PCI Video Decoder
3.1 Input Interface
100600B
Conexant
3-3
3.1.3 Flash A/D Converters
The Fusion 878A uses two on-chip flash A/D converters to digitize the video
signals. YREF+, CREF+ and YREF–, CREF– are the respective top and bottom
of the internal resistor ladders.
The input video is always AC-coupled to the decoder. CREF– and YREF– are
connected to analog ground. Voltage levels for YREF+ and CREF+ are controlled
by the gain control circuitry. If the input video momentarily exceeds the
corresponding REF+ voltage, it is indicated by LOF and COF in the DSTATUS
register.
YREF+ and CREF+ are internally connected to REFP. CREF– is internally
connected to AGND. YREF– is externally connected to AGND via YREFN.
3.1.4 A/D Clamping
An internally generated clamp control signal clamps the inputs of the
A/D converter for DC restoration of the video signals. Clamping for both the Y
and C analog inputs occurs within the horizontal sync tip. The Y input is always
restored to ground while the C input is always restored to REFP/2.
3.1.5 Power-up Operation
Upon power-up, the status of the Fusion 878A’s registers is indeterminate. The
RST signal must be asserted to set the register bits to their default values. Upon
reset, the Fusion 878A defaults to NTSC-M format.
3.1.6 Automatic Gain Controls
The Fusion 878A controls the voltage for the top of the reference ladder for each
A/D. The automatic gain control adjusts the REFP, YREF+, and CREF+ voltage
levels until the back porch sampling of the Y video input, as controlled by
ADELAY, generates a digital code 0x38 from the A/D.
3.1.7 Crystal Inputs and Clock Generation
The Fusion 878A includes an internal phase lock loop (PLL) that may be used to
decode NTSC and PAL using only a single crystal. The clock signal interface
consists of a pair of I/O pins (XTI and XTO) connected to a 28.63636 MHz
(8 × NTSC Fsc) crystal. Either fundamental or third harmonic crystals may be
used. When using the PLL, a 28.63636 MHz, 50 ppm, fundamental (or third
overtone) crystal must be connected across XTI and XT0. Alternately, a
single-ended oscillator can be connected to XTI.
Содержание Fusion 878A
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