
May 2005
23
M9999-051305
KS8995XA
Micrel, Inc.
Broadcast Storm Protection
The KS8995XA has an intelligent option to protect the switch system from receiving too many broadcast packets. Broadcast
packets will be forwarded to all ports except the source port, and thus use too many switch resources (bandwidth and available
space in transmit queues). The KS8995XA 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 zero, and the rate
limit mechanism starts to count the number of bytes during the interval. The rate definition is described in Register 6 and
Register 7. The default setting for registers 6 and 7 is 0x4A, which is 74 decimal. This is equal to a rate of 1%, calculated as
follows:
148,800 frames/sec × 50ms/interval × 1% = 74 frames/interval (approx.) = 0x4A
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 KS8995XA provides two such interfaces. The MII-P5 interface is used to connect
to 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. The table below describes the signals used in the
MII-P5 interface.
The MII-P5 interface operates in PHY mode only, while the MII-SW interface operates in either MAC mode or PHY mode. These
interfaces are nibble-wide data interfaces and therefore run at 1/4 the network bit rate (not encoded). Additional signals on the
transmit side indicate when data is valid or when an error occurs during transmission. Likewise, the receive side has indicators
that convey when the data is valid and without physical layer errors. For half-duplex operation, there is a signal that indicates
a collision has occurred during transmission.
Note that the signal MRXER is not provided on the MII-SW interface for PHY mode operation and the signal MTXER is not
provided on the MII-SW interface for MAC mode operation. Normally MRXER would indicate a receive error coming from the
physical layer device. MTXER would indicate a transmit error from the MAC device. These signals are not appropriate for this
configuration. For PHY mode operation, if the device interfacing with the KS8995XA has an MRXER pin, it should be tied low.
For MAC mode operation, if the device interfacing with the KS8995XA has an MTXER pin, it should be tied low.
MII Signal
Description
KS8995XA Signal
MTXEN
Transmit enable
PMTXEN
MTXER
Transmit error
PMTXER
MTXD3
Transmit data bit 3
PMTXD[3]
MTXD2
Transmit data bit 2
PMTXD[2]
MTXD1
Transmit data bit 1
PMTXD[1]
MTXD0
Transmit data bit 0
PMTXD[0]
MTXC
Transmit clock
PMTXC
MCOL
Collision detection
PCOL
MCRS
Carrier sense
PCRS
MRXDV
Receive data valid
PMRXDV
MRXER
Receive error
PMRXER
MRXD3
Receive data bit 3
PMRXD[3]
MRXD2
Receive data bit 2
PMRXD[2]
MRXD1
Receive data bit 1
PMRXD[1]
MRXD0
Receive data bit 0
PMRXD[0]
MRXC
Receive clock
PMRXC
MDC
Management data clock
MDC
MDIO
Management data I/O
MDIO
Table 1. MII – P5 Signals (PHY Mode)