CS8900A
Crystal LAN™ ISA Ethernet Controller
Host Enters Interrupt Routine
No
RxDMA
Process other events
that caused interrupt
Frame
bit set?
Yes
Read the DMA frame Count (CDMA
(PacketPage base + 0028h)
)
Yes
CDMA
Process other events
that caused interrupt
= 0 ?
No
Process the
DMA Frames
C
Figure 25. RxDMA Only Operation
5.5.2 Configuring the CS8900A for Auto-Switch
5.5.3 Auto-Switch DMA Operation
DMA
Whenever a frame begins to be received in Auto-
Switch DMA mode, the CS8900A checks to see if
there is enough on-chip buffer space to store a max-
imum length frame. If there is, the incoming frame
is pre-processed and buffered as normal. If there
isn’t, the CS8900A’s MAC engine compares the
frame’s Destination Address (DA) to the criteria
programmed into the DA filter. If the incoming DA
fails the DA filter, the frame is discarded. If the DA
passes the DA filter, the CS8900A automatically
switches to DMA mode and starts transferring the
frame(s) currently being held in the on-chip buffer
into host memory. This frees up buffer space for the
incoming frame.
Auto-Switch DMA mode requires the same config-
uration as Receive-DMA-only mode, with one ex-
ception: the AutoRxDMAE bit (Register 3,
RxCFG, Bit A) must be set, and the RxDMAonly
bit (Register 3, RxCFG, Bit 9) must be clear (see
Section 5.4 on page 90, Configuring the CS8900A
for DMA Operation). In Auto-Switch DMA mode,
the CS8900A operates in non-DMA mode if possi-
ble, only switching to slave DMA if necessary.
Note that if the AutoRxDMAE bit and the RxDM-
Aonly bit (Register 3, RxCFG, bit 9) are both set,
the CS8900A uses DMA for all receive frames.
CIRRUS LOGIC PRODUCT DATA SHEET
94
DS271PP3
CIRRUS [ CIRRUS LOGIC ]
