LAN9730 datasheet(30/222 Pages) MICROCHIP

30 / 222 page

LAN9730/LAN9730i

DS00001946A-page 30

 2012-2015 Microchip Technology Inc.

4.3.1.1

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 Hardware Configuration Register (HW_CFG) for details). The pad-

ding 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 bytes). 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.

If the UTX receives a Bulk-In Token when the RX FIFO is empty, it will transmit a ZLP.

FIGURE 4-1:

MEF USB ENCAPSULATION

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.

Note:

Any unused bytes that were added to the last DWORD of a frame are not counted in the length field of the

RX Status Word.

Note:

The host ignores unused bytes that exist in the first DWORD and last words of an Ethernet frame.

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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