This section includes figures that show GMII reception. In these figures, the clock is not labeled. The source of this clock signal varies depending on the options used when the core is generated.
Normal Frame Reception
The timing of normal inbound frame transfer is shown in the following figure. This shows that Ethernet frame reception is preceded by a preamble field. The IEEE 802.3 specification (refer to the Clause 35) allows for up to all of the seven preamble bytes that proceed the Start of Frame Delimiter (SFD) to be lost in the network. The SFD is always present in well-formed frames.
Normal Frame Reception with Extension Field
In accordance with the IEEE 802.3 specification, state machines for the
1000BASE-XPCS, gmii_rx_er can be driven High
following reception of the end frame with gmii_rxd[7:0] containing the hexadecimal value of 0x0F to signal carrier extension. This is shown in the
following figure. This is not an error condition and can occur even for full-duplex
frames.
Frame Reception with Errors
The signal gmii_rx_er when asserted within
the assertion window signals that a frame was received with a detected error (refer
to the following figure). In addition, a late error can also be detected during the
Carrier Extension interval. This is indicated by gmii_rxd[7:0] containing the hexadecimal value 0x1F, also shown in the following figure.
False Carrier
The following figure shows the GMII signaling for a False Carrier condition. False Carrier is asserted by the core in response to certain error conditions, such as a frame with a corrupted start code, or random noise.