LAN83C175 Datasheet, PDF (20/92 Pages) SMSC Corporation – Ethernet CARDBUS Integrated Controller With Modem Support

been copied. If the DMA reads a descriptor
with the ownership bit set to 0, it will clear the
RXQUEUED bit (and set the receive queue
empty interrupt) and wait for a new descriptor
to be queued. In fragment list mode, the
receive DMA always expects the fragment list
to contain enough buffer space for the entire
frame.
If all the buffers in the fragment list are filled
before the copy is finished, then the DMA will
abort the copy and set the fragment list error
bit in the PRSTAT register. The DMA receive
status will be posted to the descriptor for that
frame and the RXQUEUED bit will be cleared.
If early receive is enabled, the network portion
of the receive status may not yet be valid, as
indicated by the RSV bit posted in the status.
The software driver may poll the RSV bit in the
interrupt status register, and when it returns a
1 read the receive status from PRSTAT. The
software may attempt to re-copy the frame by
setting the RXQUEUED bit again, or may
discard the frame by setting the NEXTFRAME
bit before or simultaneous to setting
RXQUEUED. If RXQUEUED is set after or
along with NEXTFRAME, the DMA will begin
to copy the next frame (if any) in the receive
buffer.
Note: The DMA rounds the number of bytes
copied up to the nearest dword.
If the receive buffer does not start on a dword
boundary, then the number of bytes in the
receive buffer may be slightly less (up to 3
bytes) than the receive copy threshold when
the interrupt is generated.
Adding Receive Buffers to the Pool
The software driver adds buffers to the pool by
writing the appropriate descriptors and setting
their ownership bit to 1. If the receive DMA
has stopped (RXQUEUED is cleared), then the
software must set the RXQUEUED bit to
queue the new descriptors. The RXQUEUED
bit may be set at any time, even if the receive
DMA is still active.
Receive Lookahead Method
When this buffering method is used, the
LAN83C175 first copies only the header of a
frame into host memory, and then waits for a
queue from the software driver before copying the
rest of the frame. The software usually specifies
the final destination of the frame data with a
fragment list. The advantage to this buffering
method is that the LAN83C175 may copy frame
data to its final destination instead of a temporary
buffer space, so the software driver is not
required to re-copy the data from one host
memory location to another.
In receive lookahead mode, frames are usually
copied into the host memory one at a time, and a
handshake is performed between the software
driver and the LAN83C175 during each frame.
The handshake is performed using the
RXQUEUED and NEXTFRAME bits in the
COMMAND register, and the receive copy
complete (RCC) and header copy complete (HCC)
interrupts. The control bits in the receive
descriptors are also used to direct the receive
DMA.
The software driver begins by setting up a buffer
for the header of the first frame and setting
RXQUEUED. The HEADER control bit in the
descriptor should be set and the FRAGLIST bit
should be cleared. The buffer address pointer
and length are specified directly in the descriptor.
When a frame is received, the receive DMA
begins copying the beginning of the frame into
the header buffer until the buffer is full, or until the
entire frame has been copied. The copy may
begin before the entire frame has been received if
early receive is enabled. When the header copy
is complete, the receive DMA status will then be
posted to the descriptor for the header buffer, and
the header copy complete interrupt will be set. If
reception from the network has completed, then
the network portion of the posted status will be
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