2.0 Functional Description
Fusion 878A
2.12 DMA Controller
PCI Video Decoder
2-44
Conexant
100600B
2.12.5 Executing Instructions
Once the DMA controller has achieved synchronization between the FIFO and
the RISC program, it starts executing the RISC instructions. The data in the FIFO
will be aligned with the data bytes expected by the RISC instructions. The DMA
controller reads RISC instructions and performs burst writes from the FIFO.
The DMA controller can be programmed to wait for 4, 8, 16, or 32 DWORDs
in the FIFO before executing a WRITE instruction. Setting this FIFO trigger
point optimizes the bus efficiency by not allowing the DMA controller to access
the bus every time a DWORD enters the FIFO. However, the FIFO trigger point is
ignored when the DMA controller is near the end of an instruction and the
number of DWORDs left to transfer is less than the number of DWORDS in the
FIFO. By allowing the instruction to complete, even if the FIFO is below its
trigger point, the RISC instructions can be flushed sooner for every scan line.
Otherwise, the DMA controller may have to wait for many scan lines before the
required number of DWORDs are present in the FIFO, especially when capturing
highly scaled down images. There may be several horizontal lines before another
DWORD enters the FIFO.
The FIFO trigger point is ignored by the DMA controller during all SKIP
instructions. In the planar mode, the trigger points for the FIFOs should be set to
the same level, even though the luma data is being stored in the Y FIFO at least
twice as fast the chroma data is being stored in the Cr and Cb FIFOs. This ensures
that the Y FIFO will be selected first to burst data onto the PCI bus.
When the initiator is disconnected from the PCI bus while in the planar mode,
it is essential to regain control of the bus as soon as possible and to deliver any
Figure 2-22. Example of Complex Clipping
System DRAM
Y
Cr
Cb
Write #Bytes @ Line 0
...
Write #B @ L40, Skip #B, Wr #B @ L40
...
SYNC VRO
Write123 #B @ Y, #B @ Cr, #B @ Cb
...
SYNC VRE
JUMP
Odd Field Prog
Packed RGB
Even Field Prog
Planar 4:2:2
CPU
Host
Bridge
Frame Buffer
Video in a Window
Dialog
Box
Graphics Controller
Fusion 878A
Family
PCI Bus
879A_027