LAN9500AI datasheet(26/213 Pages) MICROCHIP

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LAN950x

DS00001875A-page 26

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5.3.1.1

Endpoint 1 (Bulk In)

The Bulk In Endpoint is controlled by the UTX (USB Bulk In Transmitter). The UTX is responsible for encapsulating

Ethernet data into a USB Bulk In packet. Ethernet frames are retrieved from the FCT’s RX FIFO.

The UTX supports the following two modes of operation: MEF and SEF, selected via the Multiple Ethernet Frames per

USB Packet (MEF) bit of the Hardware Configuration Register (HW_CFG).

• MEF: Multiple Ethernet frames per Bulk In packet. This mode will maximize USB bus utilization by allowing multi-

ple Ethernet frames to be packed into a USB packet. Frames greater than 512 bytes are split across multiple Bulk

In packets.

• SEF: Single Ethernet frame per Bulk In packet. This mode will not maximize USB bus utilization, but can poten-

tially ease the burden on a low end host processor. Frames greater than 512 bytes are split across multiple Bulk In

packets.

Each Ethernet frame is prepended with an RX Status Word by the FCT. The status word contains the frame length that

is used by the UTX to perform the encapsulation functions. The RX Status word is generated by the RX Transaction

Layer Interface (RX TLI). The TLI resides between the MAC and the FCT.

Padding may be inserted between the RX Status Word and the Ethernet frame by the FCT. This condition exists when

the RXDOFF register has a nonzero value (refer to Section 7.3.5, "Hardware Configuration Register (HW_CFG)" for

details). The padding is implemented by the FCT barrel shifting the Ethernet frame by the specified byte offset.

In accordance with the USB protocol, the UTX terminates a burst with either a ZLP or a Bulk In packet with a size of

less than the Bulk In maximum packet size (512 for HS, 64 for FS). The ZLP is needed when the total amount of data

transmitted is a multiple of a Bulk In packet size. The UTX monitors the RX FIFO size signal from the FCT to determine

when a burst has ended.

An Ethernet frame always begins on a DWORD boundary. In MEF mode, the UTX will not concatenate the end of the

current frame and the beginning of the next frame into the same DWORD. Therefore, the last DWORD of an Ethernet

frame may have unused bytes added to ensure DWORD alignment of the next frame. The addition of pad bytes depends

on whether another frame is available for transmission after the current one. If the current frame is the last frame to be

transmitted, no pad bytes will be added, as the USB protocol allows for termination of the packet on a byte boundary.

If, however, another frame is available for transmission, the current frame will be padded out so that it ends on the

DWORD boundary. This ensures the next frame to be transmitted will start on a DWORD boundary.

FIGURE 5-1:

MEF USB ENCAPSULATION (LAN9500/LAN9500I AND LAN9500A/LAN9500AI

ONLY)

Note:

In SEF mode, a ZLP is transmitted if the Ethernet frame is the same size as a Bulk In packet, or a multiple

of the Bulk In packet size.

Status

Word

Ethernet

Frame

512 Byte USB Bulk Frame

Ethernet Frame

Status

Word

Status

Word

Ethernet

Frame

Status

Word

Ethernet

Frame

Status

Word

Ethernet Frame

Status

Word

Ethernet Frame

512 Byte USB Bulk Frame

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