am79c970 Advanced Micro Devices, am79c970 Datasheet - Page 71

• Manufacturer Part Number: am79c970
• Description: Pcnettm-pci Single-chip Ethernet Controller For Pci Local Bus
• Manufacturer: Advanced Micro Devices

Transmit Operation
The transmit operation and features of the PCnet-PCI
controller are controlled by programmable options. The
PCnet-PCI controller offers a136-byte Transmit FIFO to
provide frame buffering for increased system latency,
automatic retransmission with no FIFO reload, and
automatic transmit padding.
Transmit Function Programming
Automatic transmit features such as retry on collision,
FCS generation/transmission, and pad field insertion
can all be programmed to provide flexibility in the
(re-)transmission of messages.
Disable retry on collision (DRTY) is controlled by the
DRTY bit of the Mode register (CSR15) in the initiali-
zation block.
Automatic pad field insertion is controlled by the
APAD_XMT bit in CSR4. If APAD_XMT is set, auto-
matic pad field insertion is enabled, the DXMTFCS fea-
ture is over-ridden, and the 4-byte FCS will be added to
the transmitted frame unconditionally. If APAD_XMT is
clear, no pad field insertion will take place and runt
packet transmission is possible.
The disable FCS generation/transmission feature can
be programmed dynamically on a frame by frame basis.
See the ADD_FCS description of TMD1.
Transmit FIFO Watermark (XMTFW) in CSR80 sets the
point at which the BMU requests more data from the
transmit buffers for the FIFO. A minimum of XMTFW
empty spaces must be available in the transmit FIFO be-
fore the BMU will request the system bus in order to
transfer transmit packet data into the transmit FIFO.
It is the responsibility of upper layer software to correctly
define the actual length field contained in the message
to correspond to the total number of LLC Data bytes en-
capsulated in the packet (length field as defined in the
ISO 8802-3 (IEEE/ANSI 802.3) standard). The length
value contained in the message is not used by the
PCnet-PCI controller to compute the actual number of
pad bytes to be inserted. The PCnet-PCI controller will
1-938
1010....1010
Preamble
AMD
Bits
56
10101011
Sync
Bits
8
Figure 27. ISO 8802-3(IEEE/ANSI 802.3) Data Frame
Destination
Address
Bytes
6
P R E L I M I N A R Y
Address
Source
Am79C970
Bytes
6
Transmit Start Point (XMTSP) in CSR80 sets the point
when the transmitter actually attempts to transmit a
frame onto the media. A minimum of XMTSP bytes must
be written to the transmit FIFO for the current frame be-
fore transmission of the current frame will begin. (When
automatically padded packets are being sent, it is con-
ceivable that the XMTSP is not reached when all of the
data has been transferred to the FIFO. In this case, the
transmission will begin when all of the packet data has
been placed into the transmit FIFO.)
When the entire frame is in the FIFO, attempts at trans-
mission of preamble will commence regardless of the
value in XMTSP. The default value of XMTSP is 10b,
meaning there has to be 64 bytes in the Transmit FIFO
to start a transmission.
Automatic Pad Generation
Transmit frames can be automatically padded to extend
them to 64 data bytes (excluding preamble). This allows
the minimum frame size of 64 bytes (512 bits) for
802.3/Ethernet to be guaranteed with no software inter-
vention from the host/controlling process.
Setting the APAD_XMT bit in CSR4 enables the auto-
matic padding feature. The pad is placed between the
LLC data field and FCS field in the 802.3 frame.FCS is
always added if the frame is padded, regardless of the
state of DXMTFCS. The transmit frame will be padded
by bytes with the value of 00h. The default value of
APAD_XMT is 0; this will disable auto pad generation af-
ter H_RESET.
append pad bytes dependent on the actual number of
bits transmitted onto the network. Once the last data
byte of the frame has completed, prior to appending the
FCS, the PCnet-PCI controller will check to ensure that
544 bits have been transmitted. If not, pad bytes are
added to extend the frame size to this value, and the
FCS is then added.
Length
Bytes
2
Data
LLC
46 — 1500
Bytes
Pad
Bytes
FCS
4
18220C-29