am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 54

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

Receive frames which have a length field of 46 bytes or
greater will be passed to the host unmodified.
Since any valid Ethernet Type field value will always be
greater than a normal 802.3 Length field, the MACE de-
vice will not attempt to strip valid Ethernet frames.
Receive FCS Checking
Reception and checking of the received FCS is per-
formed automatically by the MACE device. Note that if
the Automatic Pad Stripping feature is enabled, the re-
ceived FCS will be verified against the value computed
for the incoming bit stream including pad characters, but
it will not be passed through the Receive FIFO to the
host. If an FCS error is detected, this will be reported by
the FCS bit (bit 4) in the Receive Frame Status.
Receive Status Information
The EOF indication signals that the last byte/word of
data has been passed from the FIFO for the specific
frame. This will be accompanied by a RCVINT indication
in the the Interrupt Register signaling that the Receive
Frame Status has been updated, and must be read. The
Receive Frame Status is a single location which must be
read four times to allow the four bytes of status informa-
tion associated with each frame to be read. Further data
read operations from the Receive FIFO using the Regis-
ter Address mode, will be ignored by the MACE device
(indicated by the MACE chip not returning DTV) until all
four bytes of the Receive Frame Status have been read.
Alternatively, the FIFO Direct access mode may be
54
AMD
1010....1010
Preamble
Bits
56
Increasing Time
Start of Packet
at Time= 0
10101011
SYNCH
Bits
802.3 Packet and Length Field Transmission Order
8
ADDR.
Bytes
Dest.
6
Am79C940
Bit
SRCE.
ADDR.
0
Bytes
Significant
6
Most
Byte
Note that for some network protocols, the value passed
in the Ethernet Type and/or 802.3 Length field is not
compliant with either standard and may cause
problems.
The diagram below shows the byte/bit ordering of the re-
ceived length field for an 802.3 compatible frame format.
used to read the Receive Frame Status through the Re-
ceive FIFO. In either case, the 4-byte total must be read
before additional receive data can be read from the Re-
ceive FIFO. However, the RDTREQ indication will con-
tinue to reflect the state of the Receive FIFO as normal,
regardless of whether the Receive Frame Status has
been read. DTV will not be returned when a read opera-
tion is performed on the Receive Frame Status location
and no valid status is present or ready.
Note that the Receive Frame Status can be read using
either the Register Address or FIFO Direct modes. For
additional details, see the section Receive FIFO Read.
Receive Exception Conditions
Exception conditions for frame reception 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 MACE device are basically colli-
sions within the slot time and automatic runt packet de-
letion. The MACE device will ensure that any receive
packet which experiences a collision within 512 bit times
Bit
Length
Bytes
7
2
Bit
0
Significant
Least
Byte
1-1500
Bytes
DATA
LLC
Bit
7
46-1500
Bytes
16907A-013A
Bytes
16235C-9
45-0
Pad
Bytes
FCS
4