91C94 Datasheet, PDF (53/120 Pages) SMSC Corporation – ISA/PCMCIA SINGLE CHIP ETHERNET CONTROLLER WITH RAM

101 5)
RELEASE SPECIFIC PACKET - Frees all pages allocated to the packet specified in the
PACKET NUMBER REGISTER. Should not be used for frames pending transmission.
Typically used to remove transmitted frames, after reading their completion status. Can be
used following 3) to release receive packet memory in a more flexible way than 4).
110 6) ENQUEUE PACKET NUMBER INTO TX FIFO - This is the normal method of transmitting a
packet just loaded into RAM. The packet number to be enqueued is taken from the PACKET
NUMBER REGISTER.
111 7) RESET TX FIFOs - This command will reset both TX FIFOs: The TX FIFO holding the
packet numbers awaiting transmission and the TX Completion FIFO. This command
provides a mechanism for canceling packet transmissions, and reordering or bypassing
the transmit queue. The RESET TX FIFOs command should only be used when the
transmitter is disabled. Unlike the RESET MMU command, the RESET TX FIFOs does
not release any memory.
Note 1: Only command 1) uses N2,N1,N0.
Note 2: When using the RESET TX FIFOS command, the CPU is responsible for releasing the
memory associated with outstanding packets, or re-enqueuing them. Packet numbers in the
completion FIFO can be read via the FIFO ports register before issuing the command.
Note 3: MMU commands releasing memory (commands 4 and 5) should only be issued if the
corresponding packet number has memory allocated to it.
COMMAND SEQUENCING
A second allocate command (command 1)
should not be issued until the present one has
completed. Completion is determined by reading
the FAILED bit of the allocation result register or
through the allocation interrupt.
A second release command (commands 4, 5)
should not be issued if the previous one is still
being processed. The BUSY bit indicates that a
release command is in progress. After issuing
command 5, the contents of the PNR should not
be changed until BUSY goes low. After issuing
command 4, command 3 should not be issued
until BUSY goes low.
BUSY BIT - Readable at bit 0 of the MMU
command register address. When set indicates
that MMU is still processing a release
command. When clear, MMU has already
completed last release command. BUSY and
FAILED bits are set upon the trailing edge of
command.
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