A configuration bit in the transmitter control register allows you to control the length of the interframe gap transmitted by the MAC on the physical interface. If this function is selected, the MAC exerts back pressure on the user interface to delay the transmission of the next frame until the requested number of idle cycles has elapsed. The number of idle cycles is controlled by the value on the tx_ifg_delay port seen at the start of frame transmission on the user interface. The following figure shows the MAC operating in this mode.
The minimum interframe gap supported is dependent upon the support of half-duplex operation. If half-duplex is supported, the minimum IFG possible is eight bytes of transmitted data. If the MAC only supports full-duplex operation then this reduces the minimum possible IFG to four bytes of transmitted data.
In both cases, the interframe gap used when the Interframe Gap Adjust Enable bit set to 0 is the minimum value as specified in the IEEE 802.3-2008 standard. This corresponds to 12 bytes of transmitted data on the GMMI/MII interface. The value on the tx_ifg_delay port must be equal to or larger than four or eight to have an effect as described previously.
