AM79C971VCW Advanced Micro Devices, AM79C971VCW Datasheet - Page 58

• Manufacturer Part Number: AM79C971VCW
• Description: PCnet-FAST Single-Chip Full-Duplex 10/100 Mbps Ethernet Controller for PCI Local Bus
• Manufacturer: Advanced Micro Devices

If the OWN bit of the TDTE is set, but the Start of
Packet (STP) bit is not set, the Am79C971 controller
will immediately request the bus in order to clear the
OWN bit of this descriptor. (This condition would nor-
mally be found following a late collision (LCOL) or retry
(RTRY) error that occurred in the middle of a transmit
frame chain of buffers.) After resetting the OWN bit of
this descriptor, the Am79C971 controller will again im-
mediately request the bus in order to access the next
TDTE location in the ring.
If the OWN bit is set and the buffer length is 0, the OWN
bit will be cleared. In the C-LANCE device, the buffer
length of 0 is interpreted as a 4096-byte buffer. A zero
length buffer is acceptable as long as it is not the last
buffer in a chain (STP = 0 and ENP = 1).
If the OWN bit and STP are set, then microcode control
proceeds to a routine that will enable transmit data
transfers to the FIFO. The Am79C971 controller will
look ahead to the next transmit descriptor after it has
performed at least one transmit data transfer from the
first buffer.
If the Am79C971 controller does not own the next
TDTE (i.e., the second TDTE for this frame), it will com-
plete transmission of the current buffer and update the
status of the current (first) TDTE with the BUFF and
UFLO bits being set. If DXSUFLO (CSR3, bit 6) is
cleared to 0, the underflow error will cause the transmit-
ter to be disabled (CSR0, TXON = 0). The Am79C971
controller will have to be re-initialized to restore the
transmit function. Setting DXSUFLO to 1 enables the
Am79C971 controller to gracefully recover from an un-
derflow error. The device will scan the transmit descrip-
tor ring until it finds either the start of a new frame or a
TDTE it does not own. To avoid an underflow situation
in a chained buffer transmission, the system should al-
ways set the transmit chain descriptor own bits in re-
verse order.
If the Am79C971 controller does own the second TDTE
in a chain, it will gradually empty the contents of the first
buffer (as the bytes are needed by the transmit opera-
tion), perform a single-cycle DMA transfer to update the
status of the first descriptor (clear the OWN bit in
TMD1), and then it may perform one data DMA access
on the second buffer in the chain before executing an-
other
third descriptor.)
It is imperative that the host system never reads the
TDTE OWN bits out of order. The Am79C971 controller
normally clears OWN bits in strict FIFO order. However,
the Am79C971 controller can queue up to two frames
in the transmit FIFO. When the second frame uses
buffer chaining, the Am79C971 controller might return
ownership out of normal FIFO order. The OWN bit for
last (and maybe only) buffer of the first frame is not
cleared until transmission is completed. During the
58
lookahead operation. (i.e., a lookahead to the
Am79C971
transmission the Am79C971 controller will read in buff-
ers for the next frame and clear their OWN bits for all
but the last one. The first and all intermediate buffers of
the second frame can have their OWN bits cleared be-
fore the Am79C971 controller returns ownership for the
last buffer of the first frame.
If an error occurs in the transmission before all of the
bytes of the current buffer have been transferred, trans-
mit status of the current buffer will be immediately up-
dated. If the buffer does not contain the end of packet,
the Am79C971 controller will skip over the rest of the
frame which experienced the error. This is done by re-
turning to the polling microcode where the Am79C971
controller will clear the OWN bit for all descriptors with
OWN = 1 and STP = 0 and continue in like manner until
a descriptor with OWN = 0 (no more transmit frames in
the ring) or OWN = 1 and STP = 1 (the first buffer of a
new frame) is reached.
At the end of any transmit operation, whether success-
ful or with errors, immediately following the completion
of the descriptor updates, the Am79C971 controller will
always perform another polling operation. As described
earlier, this polling operation will begin with a check of
the current RDTE, unless the Am79C971 controller al-
ready owns that descriptor. Then the Am79C971 con-
troller will poll the next TDTE. If the transmit descriptor
OWN bit has a 0 value, the Am79C971 controller will
resume incrementing the poll time counter. If the trans-
mit descriptor OWN bit has a value of 1, the Am79C971
controller will begin filling the FIFO with transmit data
and initiate a transmission. This end-of-operation poll
coupled with the TDTE lookahead operation allows the
Am79C971 controller to avoid inserting poll time counts
between successive transmit frames.
By default, whenever the Am79C971 controller com-
pletes a transmit frame (either with or without error) and
writes the status information to the current descriptor,
then the TINT bit of CSR0 is set to indicate the comple-
tion of a transmission. This causes an interrupt signal if
the IENA bit of CSR0 has been set and the TINTM bit
of CSR3 is cleared. The Am79C971 controller provides
two modes to reduce the number of transmit interrupts.
The interrupt of a successfully transmitted frame can
be suppressed by setting TINTOKD (CSR5, bit 15) to
1. Another mode, which is enabled by setting LTINTEN
(CSR5, bit 14) to 1, allows suppression of interrupts for
successful transmissions for all but the last frame in a
sequence.
Receive Descriptor Table Entry
If the Am79C971 controller does not own both the cur-
rent and the next Receive Descriptor Table Entry
(RDTE), then the Am79C971 controller will continue to
poll according to the polling sequence described
above. If the receive descriptor ring length is one, then
there is no next descriptor to be polled.