KSZ8895 INFINEON [Infineon Technologies AG], KSZ8895 Datasheet - Page 36

• Manufacturer Part Number: KSZ8895
• Description: Integrated 5-Port 10/100 Managed Ethernet Switch with MII/RMII interface
• Manufacturer: INFINEON [Infineon Technologies AG]

Micrel, Inc.
Half-Duplex Back Pressure
The KSZ8895MQ/RQ/FMQ also provides a half-duplex back pressure option (note: 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 KSZ8895MQ/RQ/FMQ sends preambles to defer the other station's transmission (carrier
sense deference). To avoid jabber and excessive deference as defined in IEEE 802.3 standard, after a certain period
of time, the KSZ8895MQ/RQ/FMQ 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:
These bits are not set as the default because this is not the IEEE standard.
Broadcast Storm Protection
The KSZ8895MQ/RQ/FMQ 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 KSZ8895MQ/RQ/FMQ 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 interval for 100BT and a 500ms interval for
10BT. At the beginning of each interval, the counter is cleared to zeroand 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:
January 2011
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Aggressive backoff (Register 3, bit 0)
No excessive collision drop (Register 4, bit 3)
Back pressure (Register 4, bit 5)
148,800 frames/sec ¥ 50ms/interval ¥ 1% = 74 frames/interval (approx.) = 0x4A
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KSZ8895MQ/RQ/FMQ
M9999-012011-1.2