KS8893ML datasheet(31/106 Pages) MICREL

31 / 106 page

Micrel

KS8893M/ML/MI

June 2005

M9999-063005

Switching Engine

The KS8893M features a high-performance switching engine to move data to and from the MACs’ packet buffers.

It operates in store and forward mode, while the efficient switching mechanism reduces overall latency.

The switching engine has a 32kB internal frame buffer. This buffer pool is shared between all three ports. There

are a total of 256 buffers available. Each buffer is sized at 128 bytes.

MAC Operation

The KS8893M strictly abides by IEEE 802.3 standards to maximize compatibility.

Inter Packet Gap (IPG)

If a frame is successfully transmitted, the 96 bits time IPG is measured between the two consecutive MTXEN. If

the current packet is experiencing collision, the 96 bits time IPG is measured from MCRS and the next MTXEN.

Back-Off Algorithm

The KS8893M implements the IEEE 802.3 standard for the binary exponential back-off algorithm, and optional

"aggressive mode" back-off. After 16 collisions, the packet is optionally dropped depending on the switch

configuration for register 4 (0x04) bit [3].

Late Collision

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

Illegal Frames

The KS8893M discards frames less than 64 bytes, and can be programmed to accept frames up to1518 bytes,

1536 bytes or 1916 bytes. These maximum frame size settings are programmed in register 4 (0x04). Since the

KS8893M supports VLAN tags, the maximum sizing is adjusted when these tags are present.

Full Duplex Flow Control

The KS8893M supports standard IEEE 802.3x flow control frames on both transmit and receive sides.

On the receive side, if the KS8893M receives a pause control frame, the KS8893M will not transmit the next

normal frame until the timer, specified in the pause control frame, expires. If another pause frame is received

before the current timer expires, the timer will be updated with the new value in the second pause frame. During

this period (while it is flow controlled), only flow control packets from the KS8893M are transmitted.

On the transmit side, the KS8893M has intelligent and efficient ways to determine when to invoke flow control.

The flow control is based on availability of the system resources, including available buffers, available transmit

queues and available receive queues.

The KS8893M will flow control a port that has just received a packet if the destination port resource is busy. The

KS8893M issues a flow control frame (XOFF), containing the maximum pause time defined by the IEEE 802.3x

standard. Once the resource is freed up, the KS8893M sends out the other flow control frame (XON) 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.

The KS8893M flow controls all ports if the receive queue becomes full.

Half-Duplex Backpressure

A half-duplex backpressure option (not in IEEE 802.3 standards) is also provided. The activation and deactivation

conditions are the same as full duplex flow control. If backpressure is required, the KS8893M 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 KS8893M

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 backpressure situation, the carrier sense type backpressure is interrupted and those

packets are transmitted instead. If there are no additional packets to send, carrier sense type backpressure is

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