LAN9500AI datasheet(43/213 Pages) MICROCHIP

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DS00001875A-page 43

LAN950x

5.4

FIFO Controller (FCT)

The FIFO controller uses a 28 KB internal SRAM to buffer RX and TX traffic. 20 KB is allocated for received Ethernet-

USB traffic (RX buffer), while 8 KB is allocated for USB-Ethernet traffic (TX buffer).Bulk-Out packets from the USB con-

troller are directly stored into the TX buffer. The FCT is responsible for extracting Ethernet frames from the USB packet

data and passing the frames to the MAC.Ethernet Frames are directly stored into the RX buffer and become the basis

for bulk-in packets. The FCT passes the stored data to the UTX in blocks typically 512 or 64 bytes in size, depending

on the current HS/FS USB operating speed.

5.4.1

RX PATH (ETHERNET -> USB)

The 20 KB RX FIFO buffers Ethernet frames received from the TLI. The UTX extracts these frames from the FCT to

form USB Bulk In packets. The Host drivers will ultimately reassemble the Ethernet frames from the USB packets.

The FCT manages the writing of data into the RX FIFO through the use of two pointers - the rx_wr_ptr and the rx_wr_h-

d_ptr. The rx_wr_ptr is used to write Ethernet frame data into the FIFO. The rx_wr_hd_ptr points to the location prior to

the first DWORD of the frame. It is used to write the RX Status Word received from the TLI, upon completion of a frame

transaction. This status word contains status information associated with the frame and the frame transaction. Figure 5-

4 illustrates how a frame is stored in the FIFO, along with pointer usage.

When the RX TLI signals that it has Data ready, the RX TLI controller starts passing the RX packet data to the FCT. The

FCT updates the RX FIFO pointers as the data is written into the FIFO. The last transfer from the TLI is the RX Status

Word.

The FCT may insert 0 - 3 bytes at the start of the Ethernet frame. The value of the RX Data Offset (RXDOFF) field of

the Hardware Configuration Register (HW_CFG) on page 122 determines the number of bytes inserted.

TABLE 5-38:

FORMAT OF GET STATISTICS DATA STAGE (RX)

Offset

Field

00h

RX Good Frames

04h

RX CRC Errors

08h

RX Runt Frame Errors

0Ch

RX Alignment Errors

10h

RX Frame Too Long Error

14h

RX Later Collision Error

18h

RX Bad Frames

1Ch

RX FIFO Dropped Frames

TABLE 5-39:

FORMAT OF GET STATISTICS DATA STAGE (TX)

Offset

Field

00h

TX Good Frames

04h

TX Pause Frames

08h

TX Single Collisions

0Ch

TX Multiple Collisions

10h

TX Excessive Collision Errors

14h

TX Late Collision Errors

18h

TX Buffer Underrun Errors

1Ch

TX Excessive Deferral Errors

20h

TX Carrier Errors

24h

TX Bad Frames

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