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2.3. Avalon-ST Video Operation
Intel recommends that every video frame (or field, in the case of interlaced video) is preceded by a control packet. User packets may be presented in any order and may be re-ordered by some configurations of the VIP IP cores (e.g. the Deinterlacer II IP core when configured with 1 field of buffering). However, Intel recommends that the user packets precede the control packet.
The VIP IP cores always transmit a control packet before any video packet, and the user packets either follow or precede this control packet, depending upon the function of the IP core. When a VIP IP core receives an Avalon-ST Video control packet, the IP core decodes the height, width, and interlacing information from that packet and interprets any following Avalon-ST Video packets as being video of that format until it receives another control packet.
Most IP cores handle user packets, simply passing them through, or in the case of the Frame Buffer II IP core, writing and then reading them to memory. For IP cores that change the number of bits per symbol or symbols per pixel, additional padding is introduced to the user data.
All IP cores transmit a control packet before sending a video packet, even if no control packet has been received.
Stalling behavior (behavior when either a core is ready but there is no valid input data, or when a core has valid output data but the receiving core is not ready to receive it) varies according to the different cores. However, stalls propagate up and down the pipeline except where they can be absorbed through buffering within the cores themselves.
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