Datasheet

KSZ8863MLL/FLL/RLL
DS00002335B-page 20  2017 Microchip Technology Inc.
3.3.7.3 Late Collision
If a transmit packet experiences collisions after 512 bit times of the transmission, the packet is dropped.
3.3.7.4 Illegal Frames
KSZ8863MLL/FLL/RLL discards frames less than 64 bytes and can be programmed to accept frames up to 1518 bytes,
1536 bytes, or 1916 bytes. These maximum frame size settings are programmed in register 4 (0x04). Since
KSZ8863MLL/FLL/RLL supports VLAN tags, the maximum sizing is adjusted when these tags are present.
3.3.7.5 Full-Duplex Flow Control
KSZ8863MLL/FLL/RLL supports the standard IEEE 802.3x flow control frames on both transmit and receive sides.
On the receive side, if KSZ8863MLL/FLL/RLL receives a pause control frame, KSZ8863MLL/FLL/RLL does 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 is updated with the new value in the second pause frame. During this period
(while it is flow controlled), only flow control packets from KSZ8863MLL/FLL/RLL are transmitted.
On the transmit side, KSZ8863MLL/FLL/RLL 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.
KSZ8863MLL/FLL/RLL will flow control a port that has just received a packet if the destination port resource is busy.
KSZ8863MLL/FLL/RLL issues a flow control frame (XOFF) containing the maximum pause time defined by the IEEE
802.3x standard. Once the resource is freed up, KSZ8863MLL/FLL/RLL 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.
KSZ8863MLL/FLL/RLL flow controls all ports if the receive queue becomes full.
3.3.7.6 Half-Duplex Backpressure
A half-duplex backpressure option (not in IEEE 802.3 standards) is also provided. The activation and deactivation con-
ditions are the same as a full-duplex flow control. If backpressure is required, KSZ8863MLL/FLL/RLL sends preambles
to defer the other stations' transmission (carrier sense deference).
To avoid jabber and excessive deference (as defined in the IEEE 802.3 standard), after a certain time, KSZ8863MLL/
FLL/RLL 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 type backpressure is activated again
until switch resources free up. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense
is generated immediately, thus reducing the chance of further collisions and maintaining carrier sense to prevent packet
reception.
To ensure that no packet is lost in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable the following:
• Aggressive back-off (register 3 (0x03), bit [0])
• No excessive collision drop (register 4 (0x04), bit [3])
Note that these bits are not set as defaults because it is not the IEEE standard.
3.3.7.7 Broadcast Storm Protection
KSZ8863MLL/FLL/RLL has an intelligent option to protect the switch system from receiving too many broadcast pack-
ets. As the broadcast packets are forwarded to all ports except the source port, an excessive number of switch
resources (bandwidth and available space in transmit queues) may be utilized. KSZ8863MLL/FLL/RLL can opt to
include “multicast packets” for storm control. The broadcast storm rate parameters are programmed globally and can
be enabled or disabled on a per-port basis. The rate is based on a 67 ms interval for 100BT and a 500 ms interval for
10BT. At the beginning of each interval, the counter is cleared to zero, and the rate limit mechanism starts to count the
number of bytes during the interval. The rate definition is described in register 6 (0x06) and 7 (0x07). The default setting
is 0x63 (99 decimal). This is equal to a rate of 1%, calculated as follows:
148,800 frames/sec × 67 ms/interval × 1% = 99 frames/interval (approx.) = 0x63
Note: The 148,800 frames/sec is based on 64-byte block of packets in 100BASE-TX with 12 bytes of IPG and 8 bytes
of preamble between two packets.