KSZ9567RTXI-TR datasheet(26/232 Pages) MICROCHIP

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KSZ9567R

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The receive MAC accepts data via the integrated PHY or via the MII/RMII/RGMII interface. It decodes the data bytes,

strips off the preamble and SFD of each frame. The destination and source addresses and VLAN tag are extracted for

use in filtering and address/ID lookup, and the MAC also calculates the CRC of the received frame, which is compared

to the FCS field. The MAC can discard frames that are the wrong size, that have an FCS error, or when the source MAC

address matches the Switch MAC address.

The receive MAC also implements the Wake on LAN (WoL) feature. This system power saving feature is described in

detail in the Section 4.10, "Power Management".

MIB statistics are collected in both receive and transmit directions.

4.3.2

INTER-PACKET GAP (IPG)

If a frame is successfully transmitted, then the minimum 96-bit time for IPG is specified as being between two consec-

utive packets. If the current packet is experiencing collisions, the minimum 96-bit time for IPG is specified as being from

carrier sense (CRS) to the next transmit packet.

4.3.3

BACK-OFF ALGORITHM

The device implements the IEEE standard 802.3 binary exponential back-off algorithm in half-duplex mode. After 16

consecutive collisions, the packet is dropped.

4.3.4

LATE COLLISION

If a transmit packet experiences collisions after 512 bit times of the transmission, the packet is dropped.

4.3.5

LEGAL PACKET SIZE

On all ports, the device discards received packets smaller than 64 bytes (excluding VLAN tag, including FCS) or larger

than the maximum size. The default maximum size is the IEEE standard of 1518 bytes, but the device can be configured

to accept jumbo packets up to 9000 bytes. Jumbo packet traffic on multiple ports can stress switch resources and cause

activation of flow control.

4.3.6

FLOW CONTROL

The device supports standard MAC Control PAUSE (802.3x flow control) frames in both the transmit and receive direc-

tions for full-duplex connections.

In the receive direction, if a PAUSE control frame is received on any port, the device will not transmit the next normal

frame on that port until the timer, specified in the PAUSE control frame, expires. If another PAUSE frame is received

before the current timer expires, the timer will then update with the new value in the second PAUSE frame. During this

period (while it is flow controlled), only flow control packets from the device are transmitted.

In the transmit direction, the device has intelligent and efficient ways to determine when to invoke flow control and send

PAUSE frames. The flow control is based on availability of the system resources, including available buffers and avail-

able transmit queues.

The device issues a PAUSE frame containing the maximum pause time defined in IEEE standard 802.3x. Once the

resource is freed up, the device sends out another flow control frame with zero pause time to turn off the flow control

(turn on transmission to the port). A hysteresis feature is provided to prevent the flow control mechanism from being

constantly activated and deactivated.

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.

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