This section includes figures that illustrate GMII transmission. In these figures the clock is not labeled. The source of this clock signal varies, depending on the options selected when the core is generated.
Normal Frame Transmission
Normal outbound frame transfer timing is shown in the following
figure. This figure shows that an Ethernet frame is proceeded by an 8-byte preamble
field (inclusive of the Start of Frame Delimiter (SFD)), and completed with a 4-byte
Frame Check Sequence (FCS) field. This frame is created by the MAC connected to the
other end of the GMII. The PCS logic itself does not recognize the different fields
within a frame and treats any value placed on
gmii_txd[7:0] within the
assertion window as data.
A corrupted frame transfer is shown in the following figure. An error
can be injected into the frame by asserting
gmii_tx_er at any point during the
gmii_tx_en assertion window. The core ensures that all errors are
propagated through both transmit and receive paths so that the error is eventually
detected by the link partner.