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KSZ8765CLXCC View Datasheet(PDF) - Micrel

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KSZ8765CLXCC Datasheet PDF : 153 Pages
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Micrel, Inc.
KSZ8765CLX
Half-Duplex Back Pressure
The KSZ8765CLX also provides a half-duplex back pressure option (this is not in IEEE 802.3 standards). The activation
and deactivation conditions are the same as the ones given for full-duplex mode. If back pressure is required, the
KSZ8765CLX sends preambles to defer the other station's transmission (carrier sense deference). To avoid jabber and
excessive deference as defined in IEEE 802.3 standards, after a certain period of time, the KSZ8765CLX discontinues
carrier sense but raises it quickly after it drops packets to inhibit other transmissions. This short silent time (no carrier
sense) is to prevent other stations from sending out packets and keeps other stations in a carrier-sense-deferred state. If
the port has packets to send during a back pressure situation, the carrier-sense-type back pressure is interrupted and
those packets are transmitted instead. If there are no more packets to send, carrier-sense-type back pressure becomes
active again until switch resources are free. If a collision occurs, the binary exponential backoff algorithm is skipped and
carrier sense is generated immediately, reducing the chance of further colliding and maintaining carrier sense to prevent
reception of packets. To ensure no packet loss in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable
the following:
Aggressive backoff (Register 3, bit [0])
No excessive collision drop (Register 4, bit [3])
Back pressure (Register 4, bit [5])
These bits are not set as the default because this is not the IEEE standard.
Broadcast Storm Protection
The KSZ8765CLX has an intelligent option to protect the switch system from receiving too many broadcast packets.
Broadcast packets are normally forwarded to all ports except the source port and thus use too many switch resources
(bandwidth and available space in transmit queues). The KSZ8765CLX has the option to include multicast packets for
storm control. The broadcast storm rate parameters are programmed globally and can be enabled or disabled on a per
port basis. The rate is based on a 50ms (0.05s) interval for 100BT and a 500ms (0.5s) interval for 10BT. At the beginning
of each interval, the counter is cleared to zero and the rate limit mechanism starts to count the number of bytes during the
interval. The rate definition is described in Registers 6 and 7. The default setting for Registers 6 and 7 is 0x4A (74
decimal). This is equal to a rate of 1%, calculated as follows:
148,80 frames/sec × 50ms (0.05s)/interval × 1% = 74 frames/interval (approx.) = 0x4A
July 23, 2014
27
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