CX82100 Home Network Processor Data Sheet
7.8
RMAC Architecture
7.8.1
Support for the Detection of Invalid MAC Frames
As defined in the 802.3 specification, an invalid MAC frame meets at least one of the
following conditions:
•
•
•
The frame length is inconsistent with the length field.
The frame length is not an integral number of bytes.
The frame bits (excluding FCS) do not generate correct CRC value (CRC mismatch).
The 802.3 specification requires that contents of invalid frames must not be passed to
LLC. This functional requirement will be handled by software. The software will be
supplied status to distinguish valid from invalid frames. This is described as follows:
Condition 1
The RMAC hardware will not parse the type/length field. In the case of a valid frame, the
hardware will infer the length based on MII signaling, and pass the length (the FL field)
as part of the "status qword" (see Section 7.8.5). For invalid frames, the length
information may or may not be available to the software. In the case where the
type/length field is type, neither hardware nor software will detect this invalid condition.
If the type/length field is length, the software will detect this condition. The type/length
field indicates whether the frame is in IEEE 802.3 format or Ethernet format. A field
greater than 1500 is interpreted as a type field, which defines the type of protocol of the
frame. A field smaller than or equal to 1500 is interpreted as a length field, which
indicates the number of data bytes in the frame.
Condition 2
The RMAC hardware will detect this invalid condition and report it in the status qword as
bit DB (dribble bit).
Condition 3
The RMAC hardware will detect and record this invalid condition by using a local
Management Information Base (MIB) counter, named "CRC" (bits 52-59 of the status
qword, see Table 7-8). This is an 8-bit counter which will be reset when the RMAC
hardware detects that a good packet has been read by the DMAC. It will be incremented
by one when a CRC mismatch occurs. This counter is passed to the software as part of
the status qword.
7.8.2
Support for the Reception Without Contention
The 802.3 specification requires that each receiving station is responsible for collecting
data bits from MII as long as the Carrier Sense signal is asserted. When Carrier Sense is
deasserted, the frame is truncated to a byte boundary, if necessary, and passed to Receive
Data Decapsulation for processing. Receive Data Decapsulation is required to check the
frame’s Destination Address field to decide if the frame should be received by this
station. If so, it passes the Destination Address, the Source Address, and LLC data unit to
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