CS8920A View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'CS8920A' PDF : 144 Pages View PDF
CS8920A
5.5.9 Receive DMA Operation for
RxDMA-Only Mode
In an RxDMAOnly mode, a system DMA moves
all the received frames from the on-chip memory
to an external 16- or 64-Kbyte buffer memory.
The received frame must have passed the desti-
nation address filter, and must be completely
received. Usually the DMA receive frame inter-
rupt (RxDMAiE, bit 7, Register B, BufCFG) is
set so that the CS8920A generates an interrupt
when a frame is transferred by DMA. Figure 5.7
shows how a DMA Receive Frame interrupt is
processed.
In the interrupt service routine, the BufEvent
register (register C), bit RxDMA Frame (bit 7)
indicates that one or more receive frames were
transferred using DMA. The software driver
should maintain a pointer (e.g. PDMA_START)
that will point to the beginning of a new frame.
After the CS8920A is initialized and before any
frame is received, pointer PDMA_START points to
the beginning of the DMA buffer memory area.
The first read of the DMA Frame Count, CDMA,
commits the memory covered by the CDMA
count, and the DMA cannot overwrite this com-
mitted space until the space is freed. The driver
then processes the frames described by the
CDMA count and makes a second read of the
DMA frame count. This second read frees the
buffer memory space described by the CDMA
counter.
During the frame processing, the software should
advance the PDMA_START pointer. At the end of
processing a frame, pointer PDMA_START should
Host Enters Interrupt Routine
RxDMA
No
Frame
bit set?
Process other events
that caused interrupt
Yes
Read the DMA frame Count (CDMA)
(PacketPage base + 0028h)
Yes
CDMA
=0?
No
Process the
CDMA Frames
Process other events
that caused interrupt
Figure 5.7. RxDMA Only Operation
102
DS238PP2
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