KSZ8893-MQL Micrel Semiconductor, Inc., KSZ8893-MQL Datasheet - Page 31

• Manufacturer Part Number: KSZ8893-MQL
• Description: Integrated 3-Port 10/100 Managed Switch with PHYs
• Manufacturer: Micrel Semiconductor, Inc.

Switching Engine
The KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL supports VLAN tags, the maximum sizing is adjusted when these tags are present.
Full Duplex Flow Control
The KSZ8893MQL supports standard IEEE 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KSZ8893MQL receives a pause control frame, the KSZ8893MQL 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 KSZ8893MQL are transmitted.
On the transmit side, the KSZ8893MQL 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 KSZ8893MQL will flow control a port that has just received a packet if the destination port resource is busy.
The KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL 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
KSZ8893MQL 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
Micrel
November 2005
31
KSZ8893MQL/MQLI
M9999-111705