LAN9730 datasheet(55/222 Pages) MICROCHIP

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DS00001946A-page 55

LAN9730/LAN9730i

4.4.2.4

FCT Actions

The FCT performs basic sanity checks on the correctness of the buffer configuration, as described in Section 4.4.2.5,

"TX Error Detection". Errors in this regard indicate the TX path is out of sync, which is catastrophic and requires a reini-

tialization of the TX path.

The FCT performs the following steps when extracting an Ethernet frame:

• Strip out TX Command A

• Strip out TX Command B

• Account for the byte offset at the beginning of the frame. Based upon the buffer size and DataStartOffset[1:0] field

of TX Command A, the FCT can numerically determine any unused bytes in the first and last word of the buffer.

When transferring these respective DWORDs to the TLI, the FCT adjusts the byte enables accordingly.

Unlike the write side, the read side of the TX FIFO does not need to support rewinds. Errors are reported via the Trans-

mitter Error (TXE) flag, which is visible to the host via the Interrupt Endpoint and is also set in the Interrupt Status Reg-

ister (INT_STS).

4.4.2.5

TX Error Detection

As previously stated, both TX Command A and TX Command B are required for each buffer in a given frame. TX Com-

mand B must be identical for every buffer in a given frame, with the exception of the TX Checksum Enable (CK) bit. If

the TX Command B words do not match, then the TX path is out of sync and the FCT asserts the Transmitter Error (TXE)

flag.

Similarly, the FCT numerically adds up the size of the frame’s buffers. If there is a numerical mismatch, the TX path is

out of sync and the FCT asserts the Transmitter Error (TXE) flag. The following error conditions are tracked by the FCT:

• Missing FS - The expected first buffer of a frame does not have the FS bit set.

• Unexpected FS - The FS bit is set when the total size of buffers so far opened is less than the frame size.

• Missing LS - The total size of the buffers opened is equal to or exceeds the size of the frame. The FCT expects

this buffer to have the LS bit set and it is not set.

• Unexpected LS - The LS bit is set when the aggregate total size of descriptor buffers so far opened is less than the

frame size.

• Buffer Size is Zero Error - The buffer length field is zero.

• Buffer Size Error - The total sum of the buffers received is not equal to the frame length.

Note:

When a packet is split into multiple buffers, each successive buffer’s data payload may begin on any arbi-

trary byte.

Note:

The FCT can be configured to stall the Bulk-Out pipe when a Transmit Error is detected. This is accom-

plished via the Stall Bulk-Out Pipe Disable (SBP) bit of the Hardware Configuration Register (HW_CFG).

Refer to Section 6.3.5, "Hardware Configuration Register (HW_CFG)" for further details.

Note:

A TX Error is a catastrophic condition. The device should be reset in order to recover from it.

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