KSZ8795CLX
3.1.10
ON-CHIP TERMINATION AND INTERNAL BIASING
The KSZ8795CLX reduces the board cost and simplifies the board layout by using on-chip termination resistors for all
ports and RX/TX differential pairs without the external termination resistors. The combination of the on-chip termination
and the internal biasing will save more PCB spacing and power consumption, compared using external biasing and ter-
mination resistors for multiple switches, because the transformers don’t consume the power anymore. The center taps
of the transformer shouldn’t need to be tied to the analog power.
3.2
Media Access Controller (MAC) Operation
The KSZ8795CLX strictly abides by IEEE 802.3 standards to maximize compatibility.
3.2.1 INTER-PACKET GAP (IPG)
If a frame is successfully transmitted, the 96-bit time IPG is measured between the two consecutive MTXEN. If the cur-
rent packet is experiencing collision, the 96-bit time IPG is measured from MCRS and the next MTXEN.
3.2.2
BACKOFF ALGORITHM
The KSZ8795CLX 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.
3.2.3
LATE COLLISION
If a transmit packet experiences collisions after 512-bit times of the transmission, the packet will be dropped.
3.2.4
ILLEGAL FRAMES
The KSZ8795CLX discards frames less than 64 bytes and can be programmed to accept frames up to 1536 bytes in
Register 4. For special applications, the KSZ8795CLX can also be programmed to accept frames up to 2K bytes in Reg-
ister 3 Bit[6]. Since the KSZ8795CLX supports VLAN tags, the maximum sizing is adjusted when these tags are present.
3.2.5
FLOW CONTROL
The KSZ8795CLX supports standard 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KSZ8795CLX receives a pause control frame, the KSZ8795CLX 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 KSZ8795CLX will be transmitted.
On the transmit side, the KSZ8795CLX 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 KSZ8795CLX flow controls a port that has just received a packet if the destination port resource is busy. The
KSZ8795CLX issues a flow control frame (XOFF), containing the maximum pause time defined in IEEE standard
802.3x. Once the resource is freed up, the KSZ8795CLX 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 KSZ8795CLX flow controls all ports if the receive queue becomes full.
3.2.6
HALF-DUPLEX BACK PRESSURE
The KSZ8795CLX also provides a half-duplex back pressure option (note that this is not in IEEE 802.3 standards). The
activation and deactivation conditions are the same as the ones given for full-duplex mode. If back pressure is required,
the KSZ8795CLX sends preambles to defer the other station's transmission (carrier sense deference). To avoid jabber
and excessive deference as defined in IEEE 802.3 standards, after a certain period of time, the KSZ8795CLX discon-
tinues carrier sense but raises it quickly after it drops packets to inhibit other transmissions. This short silent time (no
carrier sense) is to prevent other stations from sending out packets and keeps 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 more packets to send, carrier sense-type back pressure
becomes active again until switch resources are free. If a collision occurs, the binary exponential backoff algorithm is
DS00002112A-page 18
2016 Microchip Technology Inc.
MICREL [ MICREL SEMICONDUCTOR ]
