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

Micrel, Inc.
KS8995XA
Switching Engine
The KS8995XA features a high performance switching engine to move data to and from the MAC’s packet buffers. It
operates in store and forward mode, while the efficient switching mechanism reduces overall latency.
The KS8995XA has a 64kB internal frame buffer. This resource is shared between all five ports. The buffer sharing
mode can be programmed through Register 2. See “Register 2.” In one mode, ports are allowed to use any free
buffers in the buffer pool.
In the second mode, each port is only allowed to use 1/5 of the total buffer pool. There are a total of 512 buffers
available. Each buffer is sized at 128B.
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
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September 2008
M9999-091508