LAN91C100 Datasheet, PDF (65/101 Pages) SMSC Corporation – FEAST™ Fast Ethernet Controller

AUTO RELEASE - When set, successful
transmit packets are not written into completion
FIFO, and their memory is released
automatically.
1) One interrupt per packet: enable TX INT,
set AUTO RELEASE=0. The software driver
can find the completion result in memory and
process the interrupt one packet at a time.
Depending on the completion code the driver
will take different actions. Note that the transmit
process is working in parallel and other
transmissions might be taking place. The
LAN91C100 is virtually queuing the packet
numbers and their status words.
In this case, the transmit interrupt service
routine can find the next packet number to be
serviced by reading the TX DONE PACKET
NUMBER at the FIFO PORTS register. This
eliminates the need for the driver to keep a list
of packet numbers being transmitted. The
numbers are queued by the LAN91C100 and
provided back to the CPU as their transmission
completes.
2) One interrupt per sequence of packets:
Enable TX EMPTY INT and TX INT, set AUTO
RELEASE=1. TX EMPTY INT is generated only
after transmitting the last packet in the FIFO.
TX INT will be set on a fatal transmit error
allowing the CPU to know that the transmit
process has stopped and therefore the FIFO will
not be emptied.
This mode has the advantage of a smaller CPU
overhead, and faster memory de-allocation.
Note that when AUTO RELEASE=1 the CPU is
not provided with the packet numbers that
completed successfully.
Note: The pointer register is shared by any
process accessing the LAN91C100 memory. In
order to allow processes to be interruptable, the
interrupting process is responsible for reading
the pointer value before modifying it, saving it,
and restoring it before returning from the
interrupt.
Typically there would be three processes using
the pointer:
1) Transmit loading (sometimes interrupt
driven)
2) Receive unloading (interrupt driven)
3) Transmit Status reading (interrupt driven).
1) and 3) also share the usage of the Packet
Number Register. Therefore saving and
restoring the PNR is also required from interrupt
service routines.
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