PM3350-SW datasheet(48/224 Pages) PMC

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ELAN 8X10

PM3350 ELAN 8 X10

DATA SHEET

PMC-970109

ISSUE 3

8 PORT ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND PMC-SIERRA, INC. CUSTOMERS ONLY

If a collision is detected (as reported by the external MAU) the MAC port forces

transmission of a jam sequence (of configurable length) and then halts transmission for

the specified backoff time, flushing its FIFO at the same time and signaling a collision.

An internal backoff timer is then loaded with the appropriate value; when the backoff

period is timed out, the MAC logic will re-attempt transmission. The computation of the

backoff period used for each collision on each MAC channel is implemented by the

Switch Processor, using a random number generator modified according to the

truncated binary exponential backoff algorithm specified by the IEEE 802.3 standard.

The Switch processor always pre-loads the backoff period that will be used for the next

collision into a backoff reload register within the MAC channel, thereby avoiding

collision backoff time variation due to firmware latencies.

Data received from the twisted-pair or coaxial cable by the MAU are converted to 8-bit

parallel bytes by the deserializer and written to the FIFO, and subsequently read out by

the DMA Controller as a stream of 32-bit words. The MAC logic performs the preamble

detection and stripping necessary to frame to the incoming data. As few as 14

consecutive bits of valid preamble (i.e., a 1010... bit pattern) may be presented to the

MAC port, along with an SFD, to guarantee proper frame synchronization; this permits

the phase locked loop settling time requirements in the physical layer transceivers to be

considerably relaxed. If an invalid pattern is detected in the preamble prior to receiving

a valid SFD, then the MAC logic rejects the entire frame, waits until the carrier sense

from the physical layer transceiver goes inactive, and begins looking for the next valid

frame.

After the carrier sense input from the external transceiver is de-asserted (indicating that

frame reception has ended), the ELAN 8x10 MAC logic blinds the receiver for a

programmable interval. During this time, transients on the carrier sense input will be

ignored (unless the MAC port was transmitting just previously) as per the IEEE 802.3

standard. The blind timer also defines the minimum tolerable interframe gap that can be

accepted by the MAC port during back-to-back frame receives.

The MAC logic also implements an internal configurable jabber counter to time out

excessively long transmissions or reception; thus the ELAN 8x10 does not require the

external MAU to provide jabber protection.

Each MAC channel implements an optional flow-control mechanism to backpressure

the channel in the case of local congestion (e.g., an out-of-buffers condition).

Backpressure is accomplished by continuously transmitting an extended jam sequence

(all-zeros) pattern, with gaps of one standard interframe spacing inserted periodically to

prevent the backpressure pattern from being interpreted as a jabber. Collisions

encountered during backpressure will not cause the MAC channel logic to back off in

the normal manner; instead, the MAC channel will send the standard jam pattern, time

out a normal interframe gap, and then resume the backpressure pattern. If, during

backpressure, the MAC channel detects that one or more frames are ready to be

transmitted over the channel by the ELAN 8x10 (e.g., a frame was received from some

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