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1.2 Video Port FIFO
The video port includes a FIFO to store data coming into or out from the video
port. The video port operates in conjunction with DMA transfers to move data
between the video port FIFO and external or on-chip memory. You can pro-
gram threshold settings so DMA events are generated when the video port
FIFO reaches a certain fullness (for capture) or goes below a certain fullness
(for display). DMAs required to service the FIFO are set up independently by
you and are key to correct operation of the video port. The FIFO size is relative-
ly large to allow time for DMAs to service the transfer requests, since devices
typically have many peripheral interfaces often including multiple video ports.
The following sections briefly describe the interaction with the DMA and differ-
ent FIFO configurations used to support the various modes of the video port.
1.2.1
DMA Interface
Video port data transfers take place using DMAs. DMA requests are based on
buffer thresholds. Since the video port does not directly source the transfer, it
can not adjust the transfer size based on buffer empty/full status. This means
the DMA transfer size is essentially fixed in the user-programmed DMA
parameter table. The preferred transfer size is often one entire line of data,
because this allows the most flexibility in terms of frame buffer line pitch (in
RAM). Some modes of operation for the highest display rates may require
more frequent DMA requests such as on a half or quarter line basis.
All requests are based on buffer thresholds. In video capture mode, DMA
requests are made whenever the number of samples in the buffer reaches the
threshold value. In order to ensure that all data from a capture field/frame gets
emptied from the buffer, the transfer size must be equal to the threshold and
the total amount of field/frame data must be a multiple of the transfer size.
For video display operation, DMA requests are made whenever there is at
least the threshold number of doublewords free in the FIFO. This means that
the transfer size must be equal to or smaller than the threshold so that it fits
into the available space. The field/frame size must still be a multiple of the
transfer size or there are pixels left in the buffer at the end of the field (which
appear at the start of the next field).
SPRU629
Video Port FIFO
Overview
1-5
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