CS8900A
Crystal LAN™ Ethernet Controller
Host Enters Interrupt Routine
No
RxDMA
Process other events
that caused interrupt
Frame
bit set?
Yes
Read the DMA frame Count (C DMA
(PacketPage base + 0028h)
)
Ye s
C
DM A
Process other events
that caused interrupt
= 0 ?
No
Process the
Fram es
C
DM A
Figure 25. RxDMA Only Operation
5.4 Auto-Switch DMA
the RxDMAonly bit (Register 3, RxCFG, Bit 9)
must be clear (see Section 5.3 on page 90,
Configuring the CS8900A for DMA Operation).
In Auto-Switch DMA mode, the CS8900A op-
erates in non-DMA mode if possible, only
switching to slave DMA if necessary.
5.4.1 Overview
The CS8900A supports a unique feature,
Auto-Switch DMA, that allows it to switch be-
tween Memory or I/O mode and Receive DMA
automatically. Auto-Switch DMA allows the
CS8900A to realize the performance advan-
tages of Memory or I/O mode while minimizing
the number of missed frames that could result
due to slow processing by the host.
Note that if the AutoRxDMAE bit and the RxD-
MAonly bit (Register 3, RxCFG, bit 9) are both
set, the CS8900A uses DMA for all receive
frames.
5.4.3 Auto-Switch DMA Operation
5.4.2 Configuring the CS8900A for Auto-
Switch 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 maximum length frame. If there is,
the incoming frame is pre-processed and buff-
Auto-Switch DMA mode requires the same
configuration as Receive-DMA-only mode,
with one exception: the AutoRxDMAE bit
(Register 3, RxCFG, Bit A) must be set, and
CIRRUS LOGIC PRODUCT DATASHEET
94
DS271F4
CIRRUS [ CIRRUS LOGIC ]
