LAN83C175 Datasheet, PDF (16/92 Pages) SMSC Corporation – Ethernet CARDBUS Integrated Controller With Modem Support

The software may add transmit frames to the
queue at any time. If the transmit process is
already running (TXQUEUED may still be set),
then all of the descriptor and fragment list
fields for the new frame must be valid
BEFORE the ownership bit in the first
descriptor is set. After the descriptors are
written, the TXQUEUED bit should be set.
TXQUEUED can be written regardless of
completion status and will ensure that the
latest frame is transmitted. If the LAN83C175
reaches the end of the transmit queue before
the new frame has been added, a transmit
chain complete interrupt is generated for the
old portion of the queue and another transmit
chain complete interrupt will be generated
when the added portion completes.
Interrupting Transmit Chain
The host may interrupt the transmit chain
before all frames have been transmitted by
setting the STOP_TDMA bit in the command
register. Setting this bit forces TXQUEUED to
0. The transmit DMA will finish copying any
frame that it has already begun, and transmit
all frames that have been loaded into the
transmit ram. After the transmit DMA has
posted the status for the last frame, it will set
the transmit chain complete interrupt and
return to its idle state (exactly as if the next
frame in the queue was owned by the host). If
the DMA reads a descriptor owned by the host
while a copy is still in progress, it will set the
transmit queued empty interrupt and wait for
the descriptor to be re-queued. It will not
return to the idle state until the copy is
completed.
Transmit Buffer Full
Whenever the local transmit RAM becomes
full, the transmit DMA will wait until more
space is available before loading any more
data. Space is freed up as the transmit MTU
reads data from the local RAM and updates its
pointers. In some cases, the transmit MTU will
leave its pointers at the beginning of a frame until
it knows that the transmission will not have to be
retried. Automatic retries can occur due to
collisions or early transmit underruns.
Transmit Underrun
A transmit underrun occurs in early transmit mode
when the transmit DMA can not keep up with
transmission on the network. Data must be read
from the local RAM before it is available. Usually,
when an underrun occurs, the transmit MTU will
generate a transmit underrun (TXU) interrupt and
update its transmit status register. The transmit
DMA will continue to operate as though nothing
has happened. The software driver will be
allowed to read the transmit status value from
TXSTAT and set the "transmit underrun go"
(TXUGO) bit to tell the MTU to retry the frame.
The MTU will re-transmit the entire frame out of
the local transmit ram. When transmission has
completed successfully, the DMA will post the
transmit status for the retry to the descriptor for
that frame. Operation will continue as it normally
would for a non-underrun situation.
The only exception to this behavior is when the
transmit MTU can not automatically retry an
underrun frame. This happens when the frame
size is larger than the transmit RAM (1.5 Kbytes)
and the transmit DMA has overwritten the
beginning of the frame before the underrun
occurs. If such an event occurs, the transmit
DMA will abort the copy and reset its pointers to
the first descriptor for that frame. The DMA will
clear the TXQUEUED bit and return to its idle
state. Transmit queue empty and transmit chain
complete interrupts will be generated along with
the transmit underrun interrupt. The software
driver must set TXUGO to reset the transmit MTU
and then set TXQUEUED if it wants to retry the
frame. The frame will be re-copied from scratch
out of host memory when TXQUEUED is set.
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