am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 50

• Manufacturer Part Number: am79c940
• Description: Media Access Controller For Ethernet Mace
• Manufacturer: Advanced Micro Devices

Transmit FCS Generation
Automatic generation and transmission of FCS for a
transmit frame depends on the value of DXMTFCS (Dis-
able Transmit FCS) when the EOF is asserted indicating
the last byte/word of data for the transmit frame is being
written to the FIFO. The action of writing the last data
byte/word of the transmit frame, latches the current con-
tents of the Transmit Frame Control register, and there-
fore determines the programming of DXMTFCS for the
transmit frame. When DXMTFCS = 0 the transmitter will
generate and append the FCS to the transmitted frame.
If the automatic padding feature is invoked (APAD XMT
in Transmit Frame Control), the FCS will be appended
regardless of the state of DXMTFCS. Note that the cal-
culated FCS is transmitted most significant bit first. The
default value of DXMTFCS is 0 after hardware or soft-
ware reset.
Transmit Status Information
Although multiple transmit frames can be queued in the
Transmit FIFO, the MACE device will not permit loss of
Transmit Frame Status information. The Transmit
Frame Status and Transmit Retry Count can only be
buffered internally for a maximum of two frames. The
MACE device will therefore not commence a third trans-
mit frame, until the status from the first frame is read.
Once the Transmit Retry Count and Transmit Frame
Status for the first transmit packet is read, the MACE
device will autonomously begin the next transmit frame,
provided that a transmit frame is pending, the XMTSP
threshold has been exceeded (or the XMTFIFO is full),
the network medium is free, and the IPG time has
elapsed.
Indication of valid Transmit Frame Status can be ob-
tained by servicing the hardware interrupt and testing
the XMTINT bit in the Interrupt Register, or by polling the
XMTSV bit in the Poll register if a continuous polling
mechanism is required. If the Transmit Retry Count data
is required (for loading, diagnostic, or management in-
formation), XMTRC must be read prior to XMTFS.
Reading the XMTFS register when the XMTSV bit is set
will clear both the XMTRC and XMTFS values.
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.
50
AMD
1010....1010
Preamble
Bits
62
SYNCH
Bits
11
2
Ethernet Format Data Frame
Bytes
Addr
Dest
6
Am79C940
Bytes
Addr
Srce
Normal events which may occur and which are handled
autonomously by the MACE device are:
(a) Collisions within the slot time with automatic retry
(b) Deletion of packets due to excessive transmission
(a) The MACE device 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 Transmit
FIFO will not be overwritten until at least 64 bytes (512
bits) of data have been successfully transmitted onto the
network. This criteria will be met, regardless of whether
the transmit frame was the first (or only) frame in the
Transmit FIFO, or if the transmit frame was queued
pending completion of the preceding frame.
(b) If 16 total attempts (initial attempt plus 15 retries)
have been made to transmit the frame, the MACE de-
vice will abandon the transmit process for the particular
frame, de-assert the TDTREQ pin, report a Retry Error
(RTRY) in the Transmit Frame Status, and set the
XMTINT bit in the Interrupt Register, causing activation
of the external INTR pin providing the interrupt is
unmasked.
Once the XMTINT condition has been externally recog-
nized, the Transmit Frame Counter (XMTFC) can be
read to determine whether the tail end of the frame that
suffers the RTRY error is still in the host memory (i.e.,
when XMTFC = 0). This XMTFC read should be re-
quested before the Transmit Frame Status read since
reading the XMTFS would cause the XMTFC to decre-
ment. If the tail end of the frame is indeed still in the host
memory, the host is responsible for ensuring that the tail
end of the frame does not get written into the FIFO and
does not get transmitted as a whole frame. It is recom-
mended that the host clear the tail end of the frame from
the host memory before requesting the XMTFS read so
that after the XMTFS read, when MACE device re-as-
serts TDTREQ, the tail end of the frame does not get
written into the FIFO. The Transmit Frame Status read
will indicate that the RTRY error occurred. The read op-
eration on the Transmit Frame Status will update the
FIFO read and write pointers. If no End-of-Frame write
(EOF pin assertion) had occurred during the FIFO write
sequence, the entire transmit path will be reset (which
will update the Transmit FIFO watermark with the
6
attempts.
Bytes
Type
2
46—1500
Bytes
Data
16235C-8
Bytes
FCS
4