Specifications

Micrel, Inc.
KSZ8895MQ/RQ/FMQ
Inter-Packet Gap (IPG)
If a frame is successfully transmitted, the 96-bit time IPG is measured between the two consecutive MTXEN. If the
current packet is experiencing collision, the 96-bit time IPG is measured from MCRS and the next MTXEN.
Backoff Algorithm
The KSZ8895MQ/RQ/FMQ implements the IEEE Standard 802.3 binary exponential backoff algorithm, and optional
“aggressive mode” backoff. After 16 collisions, the packet will be optionally dropped, depending on the chip
configuration in Register 3. See “Register 3.”
Late Collision
If a transmit packet experiences collisions after 512-bit times of the transmission, the packet will be dropped.
Illegal Frames
The KSZ8895MQ/RQ/FMQ discards frames less than 64 bytes and can be programmed to accept frames up to 1536
bytes in Register 4. For special applications, the KSZ8895MQ/RQ/FMQ can also be programmed to accept frames
up to 1916 bytes in Register 4. Since the KSZ8895MQ/RQ/FMQ supports VLAN tags, the maximum sizing is
adjusted when these tags are present.
Flow Control
The KSZ8895MQ/RQ/FMQ supports standard 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KSZ8895MQ/RQ/FMQ receives a pause control frame, the KSZ8895MQ/RQ/FMQ 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 (being flow controlled), only flow control packets from the KSZ8895MQ/RQ/FMQ will be
transmitted.
On the transmit side, the KSZ8895MQ/RQ/FMQ 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 KSZ8895MQ/RQ/FMQ flow controls a port that has just received a packet if the destination port resource is
busy. The KSZ8895MQ/RQ/FMQ issues a flow control frame (XOFF), containing the maximum pause time defined in
IEEE standard 802.3x. Once the resource is freed up, the KSZ8895MQ/RQ/FMQ 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
also provided to prevent over-activation and deactivation of the flow control mechanism.
The KSZ8895MQ/RQ/FMQ flow controls all ports if the receive queue becomes full.
March 12, 2014
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Revision 1.7