PM3350-SW View Datasheet(PDF) - PMC-Sierra
Part Name
Description
MFG CO.
'PM3350-SW' PDF : 224 Pages View PDF
ELAN 8X10
DATA SHEET
PMC-970109
ISSUE 3
PM3350 ELAN 8 X10
8 PORT ETHERNET SWITCH
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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