Datasheet

Micrel, Inc. KS8995MA/FQ
October 2011 33 M9999-102611-3.0
Half-Duplex Back Pressure
The KS8995MA/FQ 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 KS8995MA/FQ 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 KS8995MA/FQ
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 areno 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 chanceof 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 KS8995MA/FQ has an intelligent option to protect the switch system from receiving too many broadcast packets.
Broadcastpackets 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 KS8995MA/FQ 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 basedon a 50ms interval for 100BT and a 500ms 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:
148,800 frames/sec × 50ms/interval × 1% = 74 frames/interval (approximately) = 0×4A frames/interval
MII Interface Operation
The media independent interface (MII) is specified by the IEEE 802.3 committee and provides a common interface
between physical layer and MAC layer devices. The KS8995MA/FQ provides two such interfaces. The MII-P5 interface is
used to connectto the fifth PHY, whereas the MII-SW interface is used to connect to the fifth MAC. Each of these MII
interfaces contains two distinct groups of signals, one for transmission and the other for receiving. Table 1 describes the
signals used in the MII-P5 interface.