KSZ9563RNXI datasheet(26/226 Pages) MICROCHIP

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KSZ9563R

DS00002419D-page 26

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4.3.7

HALF-DUPLEX BACK PRESSURE

A half-duplex back pressure option (non-IEEE 802.3 standard) is also provided. The activation and deactivation condi-

tions are the same as in full-duplex mode. If back pressure is required, the device sends preambles to defer the other

stations' transmission (carrier sense deference).

To avoid jabber and excessive deference (as defined in the 802.3 standard), after a certain time, the device discontinues

the carrier sense and then raises it again quickly. This short silent time (no carrier sense) prevents other stations from

sending out packets thus keeping 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 additional packets to send, carrier sense type back pressure is reactivated again until chip resources free

up. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense is generated immediately,

thus reducing the chance of further collision and carrier sense is maintained to prevent packet reception.

To ensure no packet loss in 10BASE-T/Te or 100BASE-TX half-duplex modes, the user must enable the following:

• No excessive collision drop (Switch MAC Control 1 Register)

• Back pressure (Port MAC Control 1 Register)

4.3.8

FLOW CONTROL AND BACK PRESSURE REGISTERS

Table 4-4 provides a list of flow control and back pressure related registers.

4.3.9

BROADCAST STORM PROTECTION

The device has an intelligent option to protect the switch system from receiving too many broadcast packets. As the

broadcast packets are forwarded to all ports except the source port, an excessive number of switch resources (band-

width and available space in transmit queues) may be utilized. The device 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 5ms interval for 1000BASE-T, a 50ms interval for 100BASE-TX and a 500ms

interval for 10BASE-T/Te. 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 control registers. The default

setting equates to a rate of 1%.

4.3.10

SELF-ADDRESS FILTERING

Received packets can be filtered (dropped) if their source address matches the device's MAC address. This feature is

useful for automatically terminating packets once they have traversed a ring network and returned to their source. It can

be enabled on a per-port basis via the Switch Lookup Engine Control 1 Register and Port Control 2 Register.

TABLE 4-4:

FLOW CONTROL AND BACK PRESSURE REGISTERS

Registers

Description

Switch MAC Address 0 Register

through

Switch MAC Address 5 Register

Switch's MAC address, used as source address of PAUSE control

frames

Switch MAC Control 0 Register

“Aggressive back-off” enable

Switch MAC Control 1 Register

BP mode, “Fair mode” enable, “no excessive collision drop” enable

Switch MAC Control 4 Register

Pass PAUSE control frames

Port Status Register

Flow control enable (per port)

PHY Auto-Negotiation Advertisement Register PHY - flow control advertisement (per port)

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