AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 76

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

The 544 bit count is derived from the following :
A minimum length transmit frame from the PCnet-PCI II
controller will therefore be 576 bits, after the FCS
is appended.
Transmit FCS Generation
Automatic generation and transmission of FCS for a
transmit frame depends on the value of DXMTFCS
(CSR15, bit 3). If DXMTFCS is cleared to ZERO, the
transmitter will generate and append the FCS to the
transmitted frame. If the automatic padding feature is in-
voked (APAD_XMT is set in CSR4), the FCS will be ap-
pended by the PCnet-PCI II controller regardless of the
state of DXMTFCS or ADD_FCS/NO_FCS (TMD1, bit
29). Note that the calculated FCS is transmitted most
significant bit first. The default value of DXMTFCS is 0
after H_RESET.
ADD_FCS (TMD1, bit 29) allows the automatic genera-
tion and transmission of FCS on a frame by frame basis.
DXMTFCS should be cleared to ZERO in this mode. To
generate FCS for a frame, ADD_FCS must be set in the
first descriptor of a frame (STP is set to ONE). Note that
bit 29 of TMD1 has the function of ADD_FCS if
SWSTYLE (BCR20, bits 7–0) is programmed to ZERO,
TWO or THREE.
When SWSTYLE is set to ONE for ILACC backwards
compatibility, bit 29 of TMD1 changes its function to
NO_FCS. When DXMTFCS is cleared to ZERO and
NO_FCS is set to ONE in the last descriptor of a frame
(ENP is set to ONE), the PCnet-PCI II controller will not
generate and append an FCS to a transmit frame.
76
Minimum frame size (excluding 64 bytes 512 bits
preamble/SFD, including FCS)
Preamble/SFD size
FCS size
At the point that FCS is to be appended, the transmit-
ted frame should contain:
Preamble/SFD + (Min Frame Size – FCS)
64 + (512 – 32) = 544 bits
1010....1010
AMD
Preamble
Bits
56
10101011
SFD
Bits
8
Figure 34. ISO 8802-3 (IEEE/ANSI 802.3) Data Frame
Destination
Address
8 bytes
4 bytes
Bytes
6
64 bits
32 bits
P R E L I M I N A R Y
Am79C970A
Address
Source
Bytes
6
Transmit Exception Conditions
Exception conditions for frame transmission fall into two
distinct categories. Those which are the result of normal
network operation, and those which occur due to abnor-
mal network and/or host related events.
Normal events which may occur and which are handled
autonomously by the PCnet-PCI II controller include col-
lisions within the slot time with automatic retry. The
PCnet-PCI II controller will ensure that collisions which
occur within 512 bit times from the start of transmission
(including preamble) will be automatically retried with no
host intervention. The transmit FIFO ensures this by
guaranteeing that data contained within the FIFO will
not be overwritten until at least 64 bytes (512 bits) of pre-
amble plus address, length and data fields have been
transmitted onto the network without encountering a col-
lision. Note that if DRTY (CSR15, bit 5) is set to ONE or if
the network interface is operating in full-duplex mode,
no collision handling is required, and any byte of frame
data in the FIFO can be overwritten as soon as it
is transmitted.
If 16 total attempts (initial attempt plus 15 retries) fail, the
PCnet-PCI II controller sets the RTRY bit in the current
transmit TDTE in host memory (TMD2), gives up
ownership (resets the OWN bit to ZERO) for this frame,
and processes the next frame in the transmit ring
for transmission.
Abnormal network conditions include:
These conditions should not occur on a correctly config-
ured 802.3 network operating in half-duplex mode, and
will be reported if they do. None of these conditions will
occur on a network operating in full-duplex mode. (See
the section “Full-Duplex Operation” for more detail.)
When an error occurs in the middle of a multi-buffer
frame transmission, the error status will be written in the
Loss of carrier.
Late collision.
SQE Test Error. (Does not apply to 10BASE-T port.)
Length
Bytes
2
Data
LLC
46 – 1500
Bytes
Pad
Bytes
FCS
4
19436A-37