ACD82124 Datasheet, PDF (6/48 Pages) List of Unclassifed Manufacturers – 24 Ports 10/100 Fast Ethernet Switch Controller

A port’s MAC address register is cleared on power-
up, hardware reset, or when the port enters into Link
Fail state. If the SA aging option is enabled (Register-
16 bit 4), the learned SA will be cleared if it does not
reappear within five minutes.
During the receive process, the Lookup Engine will
attempt to match the destination address with the ad-
dresses stored in the address table. If a match is found,
a link between the source port and the destination port
is established. If an external ARL is used, the ACD82124
indicates the presence of a 48-bit DA through the sta-
tus line of the ARL interface. The external ARL will use
the value of DA for address comparison and return a
result of the lookup to the ACD82124.
Frame Data
Frame data are transparent to the ACD82124. The
ACD82124 will forward the data to the destination
port(s) without interpreting the content of the frame
data field.
FCS Calculation
Each port of the ACD82124 has CRC checking logic
to verify if the received frame has a correct FCS value.
A wrong FCS value is an indication of a fragmented
frame or a frame with frame bit error. The method of
calculating the CRC value is using the following poly-
nomial,
G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11
+ x10 + x8 + x7 + x5 + x4 + x2 + x + 1
as a divider to divide the bit sequence of the incoming
frame, beginning with the first bit of the destination
address field, to the end of the data field. The result of
the calculation, which is the residue after the polyno-
mial division, is the value of the frame check sequence.
This value should be equal to the FCS field appended
at the end of the frame. If the value does not match the
FCS field of the frame, the Frame Bit Error LED of the
port will be turned on once and the packet will be
dropped.
Frame Length
During the receiving process, the MAC will monitor the
length of the received frame. Legal Ethernet frames
should have a length of not less than 64 bytes and no
more than 1518 bytes. If the carrier sense signal of a
frame is asserted for less than 76 BT, the frame is
flagged with short event error. If the length of a frame
is less then 64 bytes, the frame is flagged with runt
error.
In order to support an application where extra byte
length is required, an Extra-Long-Frame option is pro-
vided. When the Extra long frame option is enabled
(Table 12: CFG7), only frames longer than 1530 bytes
are marked with a long event error. Frame length is
measured from the first byte of DA to the last byte of
FCS.
Frame Filtering
Frames with any kind of error will be filtered. Types of
error include code error (indicated by assertion of
RXER signal), FCS error, alignment error, short event,
runt, and long event.
Any frame heading to its own source port will be fil-
tered. If external ARL is used, the ACD82124 will filter
the frame as directed by the external ARL.
If the Spanning Tree Support option is enabled, frames
containing DA equal to any reserved Bridge Manage-
ment Group Address specified in Table 3.5 of IEEE
802.1d will not be forwarded to any ports, except the
Port-23, which may receive BPDU frames. If span-
ning tree support is not enabled, frames with DA equal
to the reserved Group Address for PBDU will be broad-
casted to all ports in the same VLAN of the source
port.
Jabber Lockup Protection
If a receiving port is active continuously for more than
50,000 BT, the port is considered to be jabbering. A
jabbering port will automatically be partitioned from the
switch system in order to prevent it from impairing the
performance of the network. The partitioned port will
be re-activated as soon as the offending signal dis-
continues.
Excessive Collision
In the event that there are more than 16 consecutive
collision, the ACD82124 will reset the counter to zero
and retransmit the packet. This implementation insures
there is no packet loss even under channel capture
situation. However, ACD82124 has an option to drop
the packet on excessive collision. When this option is
enabled (Table 12: CG11), the frame will be dropped
after 16 consecutive collisions.
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