Altera 10-Gbps Ethernet MAC MegaCore Function - Manual (Page 78)

Chapter 7: Functional Description 7–11
Receive Datapath
February 2014 Altera Corporation 10-Gbps Ethernet MAC MegaCore Function User Guide
7.5. Receive Datapath
The MAC RX receives Ethernet frames from the SDR XGMII and forwards the
payload with relevant frame fields to the client after performing checks and filtering
invalid frames. Some frame fields are optionally removed from the frame before MAC
RX forwards the frame to the client.
Figure 7–8 shows the typical flow of frame through the MAC RX.
7.5.1. Minimum Inter-Packet Gap
Table 7–1 shows the minimum IPG the MAC can receive for the different interfaces.
7.5.2. XGMII Decapsulation
In the receive datapath, the MAC RX decodes the data lanes coming through the SDR
XGMII. The MAC RX expects the first byte of the receive frame to be in either lane 0
(most significant byte) or lane 4. The receive frame must also be preceded by a column
of idle bytes or an ordered set such as a local fault. A receive frame that does not
satisfy these conditions is invalid and the MAC RX drops the frame.
The MAC RX then checks the sequence of the frame. The frame must begin with a
1-byte START, 6-byte preamble, and 1-byte SFD. Otherwise, the MAC RX considers
the frame invalid and drops it. For all valid frames, the MAC RX removes the START,
preamble, SFD, and EFD bytes and ensures that the first byte of the frame aligns to
byte 0.
When you enable the preamble passthrough mode, the MAC RX only checks for the
following conditions: the frame begins with a 1-byte START and the minimum length
of the frame including the START and client-defined preamble is 12 bytes.
For frames that do not fulfill these conditions, the MAC RX considers the frames
invalid and drops them. For all valid frames, the MAC RX removes the EFD byte and
ensures that the first byte of the frame aligns to byte 0. The MAC RX forwards the
START and client-defined preamble to the client.
Figure 7–8. Typical Client Frame at Receive Interface
Client - MAC Rx Interface
(optional)
Client Frame
Destination
Addr[47:0]
Source
Addr[47:0]
Type/
Length[15:0]
Payload
[<p-1>:0]
Destination
Addr[47:0]
CRC32
[31:0]
PAD [<s>]
Source
Addr[47:0]
Client-Defined Preamble
[55:0]
(optional)
Type/
Length[15:0]
Payload
[<p-1>:0]
PAD [<s>]
CRC32
[31:0]
EFD[7:0]
Start[7:0]
Frame Length
(1) (2)
MAC Frame
SFD[7:0]
Preamble
[47:0]
Start[7:0]
Table 7–1. Minimum IPG for the MAC on the Receive Path
Interfaces Minimum IPG (Bytes)
XGMII (10 Gbps) 5
GMII (1 Gbps) 8
MII (10 Mbps and 100 Mbps) 6