KSZ8995XA Micrel Inc, KSZ8995XA Datasheet - Page 22

• Manufacturer Part Number: KSZ8995XA
• Description: IC SWITCH 10/100 5PORT 128PQFP
• Manufacturer: Micrel Inc

Specifications of KSZ8995XA

• Applications: *
• Mounting Type: Surface Mount
• Package / Case: 128-MQFP, 128-PQFP
• Number Of Primary Switch Ports: 5
• Internal Memory Buffer Size: 64
• Operating Supply Voltage (typ): 1.8/2.5/3.3V
• Fiber Support: Yes
• Integrated Led Drivers: Yes
• Phy/transceiver Interface: MII/SNI
• Power Supply Type: Analog/Digital
• Package Type: PQFP
• Data Rate (typ): 10/100Mbps
• Vlan Support: Yes
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Pin Count: 128
• Mounting: Surface Mount
• Jtag Support: No
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: 576-1642 - BOARD EVALUATION FOR KSZ8995XA
• Lead Free Status / RoHS Status: Compliant, Lead free / RoHS Compliant
• Other names: 576-1042

KS8995XA
Media Access Controller (MAC) Operation
The KS8995XA strictly abides by IEEE 802.3 standards to maximize compatibility.
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 KS8995XA implements the IEEE Std. 802.3 binary exponential back-off algorithm, and optional “aggressive mode” back
off. 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 KS8995XA discards frames less than 64 bytes and can be programmed to accept frames up to 1536 bytes in Register
4. For special applications, the KS8995XA can also be programmed to accept frames up to 1916 bytes in Register 4. Since
the KS8995XA supports VLAN tags, the maximum sizing is adjusted when these tags are present.
Flow Control
The KS8995XA supports standard 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KS8995XA receives a pause control frame, the KS8995XA 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 KS8995XA will be transmitted.
On the transmit side, the KS8995XA 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 KS8995XA will flow control a port, which just received a packet, if the destination port resource is being used up. The
KS8995XA will issue a flow control frame (XOFF), containing the maximum pause time defined in IEEE standard 802.3x. Once
the resource is freed up, the KS8995XA will send 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
activated and deactivated too many times.
The KS8995XA will flow control all ports if the receive queue becomes full.
Half-Duplex Back Pressure
A half-duplex back pressure option (note: not in 802.3 standards) is also provided. The activation and deactivation conditions
are the same as the above in full-duplex mode. If back pressure is required, the KS8995XA will send preambles to defer the
other stations’ transmission (carrier sense deference). To avoid jabber and excessive deference defined in 802.3 standard,
after a certain time it will discontinue the carrier sense but it will raise the carrier sense quickly. This short silent time (no carrier
sense) is to prevent other stations from sending out packets and keeps other stations in carrier sense deferred state. If the port
has packets to send during a back pressure situation, the carrier-sense-type back pressure will be interrupted and those
packets will be transmitted instead. If there are no more packets to send, carrier-sense-type back pressure will be active again
until switch resources are free. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense is
generated immediately, reducing the chance of further colliding and maintaining carrier sense to prevent reception of packets.
To ensure no packet loss in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable the following:
These bits are not set as the default because this is not the IEEE standard.
M9999-051305
• Aggressive backoff (register 3, bit 0)
• No excessive collision drop (register 4, bit 3)
• Back pressure (register 4, bit 5)
22
Micrel, Inc.
May 2005