LAN9500 Datasheet, PDF (27/213 Pages) SMSC Corporation – USB 2.0 to 10/100 Ethernet Controller Promiscuous mode

LAN950x
If the UTX receives a Bulk In token when the RX FIFO is empty, it will transmit a ZLP.
Note 1: 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.
2: The Host ignores unused bytes that exist in the first DWORD and last words of an Ethernet frame.
3: When using SEF mode, there will never be any unused bytes added for end alignment padding. The USB
transfer always ends on the last byte of the Ethernet frame.
4: When RX COE is enabled, the last byte would pertain to the RX COE word.
Once a decision is made to end a transfer and a short packet or ZLP has been sent, it is possible that an Ethernet frame
will arrive prior to the UTX seeing an ACK from the Host for the previous Bulk In packet. In this case, the UTX must
continue to repeat the short packet or ZLP until the ACK is received for the end of the previous transfer. The UTX must
not start a new transfer, or re-use the previous data toggle, to begin sending the next Ethernet frame until the ACK has
been received for the end of the previous transfer.
In order to more efficiently utilize USB bandwidth in MEF mode, the UTX has a mechanism for delaying the transmission
of a short packet, or ZLP. This mode entails having the UTX wait a time defined by the Bulk In Delay Register
(BULK_IN_DLY) before terminating the burst. A value of zero in this register disables this feature. By default, a delay of
34 us is used.
After the UTX transmits the last USB wPacketSize packet in a burst, the UTX will enable an internal timer. When the
internal timer is equal to the Bulk In Delay, any Bulk In data will be transmitted upon reception of the next Bulk In Token.
If enough data arrives before the timer elapses to build at least one maximum sized packet, then the UTX will transmit
this packet when it receives the next Bulk In Token. After packet transmission, the UTX will then reset its internal timer
and delay the short packet, or ZLP, transmission until the Bulk In Delay time elapses.
In the case where the FIFO is empty and a single Ethernet packet less than the USB wPacketSize has been received,
the UTX will enable its internal timer. If enough data arrives before the timer elapses to build at least one maximum sized
packet, then the UTX will transmit this packet when it receives the next Bulk In Token and will reset the internal timer.
Otherwise, the short packet, or ZLP, is sent in response to the first Bulk In Token received after the timer expires.
The UTX will NAK any Bulk In tokens while waiting for the Bulk In Delay to elapse. This NAK response is not affected
by the Bulk In Empty Response (BIR). The Bulk In Empty Response (BIR) setting only applies after the Bulk In Delay
time expires.
The UTX, via the Burst Cap Register (BURST_CAP), is capable or prematurely terminating a burst. When the amount
transmitted exceeds the value specified in this register, the UTX transmits a ZLP after the current Bulk In packet com-
pletes. The Burst Cap Register (BURST_CAP) uses units of USB packet size (512 bytes). To enable use of the Burst
Cap register, the Burst Cap Enable (BCE) bit in the Hardware Configuration Register (HW_CFG) must be set. For proper
operation, the BURST_CAP field should be set to value greater than 4 for HS mode and greater than 32 for FS mode.
Burst Cap enforcement is disabled if BURST_CAP is set to a value less than or equal to 4 for HS mode and less than
or equal to 32 for FS mode.
Whenever Burst Cap enforcement is disabled, the UTX will respond with a ZLP (when Bulk In Empty Response (BIR)
=0) or with NAK (when Bulk In Empty Response (BIR) = 1).
Whenever Burst Cap enforcement is enabled (BURST_CAP value is legal), the following holds:
• For HS Operation:
- Let BURST = BURST_CAP * 512
- The burst may terminate at BURST-4, BURST-3, BURST-2, BURST-1, or BURST bytes, or, when the RX FIFO runs out of
data. The burst is terminated with either a short USB packet or with a ZLP.
• For FS operation:
- The burst will terminate after BURST_CAP * 64 bytes.
Note: Ethernet frames are not fragmented across bursts when using Burst Cap Enforcement.
In the case of an error condition, the UTX will issue a rewind to the FCT. This occurs when the UTX completes trans-
mitting a Bulk In packet and does not receive an ACK from the Host. In this case, the next frame received by the UTX
will be another In token and the Bulk In packet is retransmitted. When the ACK is finally received, the UTX notifies the
FCT. The FCT will then advance the read head pointer to the next packet.
Note: The UTX will never stall the endpoint. The endpoint can only be stalled by the Host.
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DS00001875A-page 27