DS_8430_001
78Q8430 Data Sheet
6.10.8 Strip Padding/FCS
The strip-padding feature will remove any padding from 64-byte frames. The strip-padding feature will
also remove excess padding from frames up to 127 bytes in total length. Frames that are 128 bytes or
larger will never have any padding stripped. Padded frames that have their padding removed by the strip-
padding feature will also have their FCS field removed, even if the strip CRC feature is not enabled. If the
strip-padding feature is enabled but the strip CRC feature is not, then frames received that have no
padding removed will still have their FCS field, but frames that have padding removed will not. The CRC
is always checked even if it is removed by the strip padding feature and not the strip CRC feature. The
strip CRC feature will always remove the FCS field from all received frames.
The packet classification block snoops data bytes as they are popped from the receive FIFO by the QUE
receive controller. If the classifier determines that a frame is to be dropped, the frame error bit to the
receive producer is set and, assuming the first BLOCK of the frame has not been added, the frame is
dumped before it is appended to the QUE. If the classification block determines that a frame is to be
dropped after one or more BLOCKs have been added to the QUE then the frame is truncated at that point
and the remainder of the frame is dropped.
6.11 MAC Error Reporting
Errors reported by the MAC are communicated back to the host on a frame-by-frame basis through the
Transmit Status FIFO and Receive Status FIFO.
6.11.1 MAC Transmit Errors
6.11.1.1 Transmit FIFO Underrun Error
The 80-byte MAC transmit FIFO is capable of handling a worst-case QUE latency of 1.28 μs (128 bit
times, or 16 byte times) because 64 bytes are retained for possible retransmission after a collision. The
QUE transmit controller has higher bandwidth than the 100 Mb PHY such that a MAC transmit FIFO
under-run usually indicates a host bus latency problem. See Section 6.3.2.2 for a discussion on how to
mitigate this situation.
In the event that the transmit FIFO does under-run, the MAC aborts the transmission, sets the Underrun
bit in the transmit status and discards the remainder of the frame when it finally does arrive.
6.11.1.2 Lost Carrier Error
In half duplex mode, Carrier Sense (CRS) is monitored from the beginning of the Start Frame Delimiter
(SFD) to the last byte transmitted. A lost carrier condition indicates that CRS was never present or was
dropped during transmission (a possible network problem), but transmission is not aborted. Lost carrier
error is disabled during loop back mode. During full duplex operation, CRS is not passed to the transmit
block and lost carrier will not be asserted. Lost carrier sets the Carrier bit in the Transmit Packet Status
Register (TPSR).
6.11.1.3 Excessive Collision Error
In half duplex mode, whenever the MAC encounters a collision during transmit, it will back off, update the
collision counter and try again later. When the counter equals 16 (16 attempts all resulted in a collision)
transmission is aborted. Excessive collisions probably indicate a network problem. Excessive collision
sets the Excessive Collision bit in the Transmit Packet Status Register (TPSR).
6.11.1.4 Late Collision Error (Transmit Out-Of-Window Collision)
In a correctly configured and operating network, the controller sees a collision (if there is one) within the
first 64 bytes of data being transmitted. If a collision occurs after this time a possible network problem is
detected. Late collision sets the Late Collision bit in the Transmit Packet Status Register (TPSR) and
transmission of the packet is aborted, i.e. late collisions are not retried.
Rev. 1.2
47
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
