LAN89730 datasheet(50/218 Pages) MICROCHIP

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LAN89730

DS60001348A-page 50

 2013-2015 Microchip Technology Inc.

4.4.1.1

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 123 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

Refer to Section 4.3.1.7, "Statistics" 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.

Dropping an Ethernet frame is implemented by rewinding the received frame. A write side rewind is implemented by

setting the rx_wr_ptr to be equal to the rx_wr_hd_ptr. Similarly, a read side rewind is implemented by setting the

rx_rd_ptr to be equal to the rx_rd_hd_ptr.

For the case where the frame is dropped due to overflow, the FCT ignores the remainder of the frame. It will not begin

writing into the RX FIFO again until the next frame is received.

In the read direction, the FCT must also support rewinds for the UTX. This is needed for the case where the USB Bulk-

Out packet is not successfully received by the host and needs to be retransmitted.

4.4.1.2

RX Status Format

Table 4-40 illustrates the format of the RX Status Word.

TABLE 4-40:

RX STATUS WORD FORMAT

Bits

Description

RESERVED

Filtering Fail

When set, this bit indicates that the associated frame failed the address recognizing filtering.

29:16

Frame Length

The size, in bytes, of the corresponding received frame.

Error Status (ES)

When set, this bit indicates that the TLI has reported an error. This bit is the logical OR of bits 11, 7, 6,

1 in this status word.

RESERVED

Broadcast Frame

When set, this bit indicates that the received frame has a Broadcast address.

Length Error (LE)

When set, this bit indicates that the actual length does not match with the length/type field of the

received frame.

Runt Frame

When set, this bit indicates that frame was prematurely terminated before the collision window (64

bytes). Runt frames are passed on to the host only if the Pass Bad Frames bit MAC_CR Bit [16] is set.

Multicast Frame

When set, this bit indicates that the received frame has a Multicast address.

9:8

RESERVED

Frame Too Long

When set, this bit indicates that the frame length exceeds the maximum Ethernet specification of 1518

bytes. This is only a Frame Too Long indication and will not cause the frame reception to be truncated.

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