CS8920A View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'CS8920A' PDF : 144 Pages View PDF
CS8920A
is not the first frame to be transferred. Instead,
the oldest non-committed frame in the on-chip
buffer is the first frame to use DMA. When
DMA begins, any pending RxEvent reports in
the Interrupt Status Queue are discarded because
the host cannot process those events until the
corresponding frames have been completely
DMAed.
Auto-Switch DMA works only on entire received
frames. The CS8920A does not use Auto-Switch
DMA to transfer partial frames. Also, when a
frame has been committed (see Section 5.2.5),
the CS8920A will not switch to DMA mode un-
til the committed frame has been transferred
completely or skipped.
After a complete frame has been moved to host
memory, the CS8920A updates the DMA Start-
of-Frame register (PacketPage base + 0026h), the
DMA Frame Count register (PacketPage base +
0028h), and the DMA Byte Count register, then
sets the RxDMAFrame bit (Register C,
BufEvent, bit 7). If RxDMAiE (Register B,
BufCFG, bit 7) is set, a corresponding interrupt
occurs.
DMA Channel Speed vs. Missed Frames
When the CS8920A starts DMA, the entire old-
est, non-committed frame must be placed in host
memory before on-chip buffer space will be
freed for the next incoming frame. If the oldest
frame is relatively large, and the next incoming
frame is also large, the incoming frame may be
missed, depending on the speed of the DMA
channel. If this happens, the CS8920A will in-
crement the RxMiss counter (Register 10) and
clear any event reports (RxEvent and BufEvent)
associated with the missed frame.
Exit From DMA
When the CS8920A has activated receive DMA,
it remains in DMA mode until all of the follow-
ing are true:
104
• The host processes all RxEvent and
BufEvent reports pending in the ISQ.
• The host reads a zero value from the DMA
Frame Count register (PacketPage base +
0028h).
• The CS8920A is not in the process of trans-
ferring a frame via DMA.
Auto-Switch DMA Example
Figure 5.9 shows how the CS8920A enters and
exits Auto-Switch DMA mode.
DS238PP2
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