LAN9500 Datasheet, PDF (44/213 Pages) SMSC Corporation – USB 2.0 to 10/100 Ethernet Controller Promiscuous mode

LAN950x
A received Ethernet frame is not visible to the UTX until the complete frame, including the RX Status Word, has been
written into the RX FIFO. This is due to the fact that the frame may have to be removed via a rewind (pointer adjustment),
in case of an error. Such is the case when a FIFO overflow condition is detected as the frame is being received. The
FCT may also be configured to rewind errored frames. Please refer to Section 5.4.1.1, "RX Error Detection," on page 44
for further details.
FIGURE 5-4:
RX FIFO STORAGE
FIFO data is available for
transmit only after a
complete Ethernet frame is
received and stored.
Therefore, the RX FIFO
size will not reflect partially
received packets.
RX Ethernet
Frame 2
USB
Packet 3
RX FIFO Size
USB
Packet 2
RX Ethernet
Frame 1
USB
Packet 1 _{RX Status Word}
rx_rd_ptr
rx_rd_hd_ptr
USB
Packet 0
RX Ethernet
Frame 0
RX Status Word
rx_wr_ptr
rx_wr_hd_ptr
After the complete
Ethernet frame is written,
the size and status is
updated at the location
pointed to by the write
head pointer.
The write head pointer will
then advance to the
starting location for the
next Ethernet frame.
The read head pointer is
used for implementing
rewinds of USB packets.
Byte padding inserted by
the FCT. This amount is
determined by
RXDOFF[1:0]
Additional padding may be
inserted by the UTX.
The unused bytes in the
first and last DWORDs are
ignored by the host.
5.4.1.1 RX Error Detection
The FCT can be configured to drop Ethernet frames when certain error conditions occur. The setting of the Discard
Errored Received Ethernet Frame (DRP) bit of the Hardware Configuration Register (HW_CFG) on page 122 deter-
mines if the frame will be retained or dropped. Error conditions are indicated in the Rx Status Word. The following error
conditions are tracked by the TLI:
• CRC Error
• Collision Seen
• Frame Too Long
• Runt Frame
Please refer to Section 5.3.1.7, "Statistics," on page 36 for more details on the error conditions tracked by the device.
The FCT also drops frames when it detects a FIFO overflow condition. This occurs when the FIFO full condition occurs
while a frame is being received. The FCT also maintains a count of the number of times a FIFO overflow condition has
occurred.
DS00001875A-page 44
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