2.11.7. TX Custom PCS Interface to User Logic
The E-Tile Hard IP for Ethernet Intel FPGA IP TX client interface in custom PCS variation employs the Media Independent Interface (MII) protocol.
The client acts as a source and the TX PCS acts as a sink in the transmit direction.
|1 bit for each channel||
The TX clock for the IP core that drives the channel.
64 bits for each channel
TX MII data. Data must be in MII encoding. i_tx_mii_d[7:0] holds the first byte the IP core transmits on the Ethernet link. i_tx_mii_d holds the first bit the IP core transmits on the Ethernet link.
While the TX MII valid signal has the value of 0 or the alignment marker insertion bit signal has the value of 1, and for one additional clock cycle, you must hold the value of this signal stable. We refer to this behavior as freezing the signal value.
8 bits for each channel
|TX MII control bits. Each bit corresponds to a byte of the TX MII data signal. For example, i_tx_mii_c corresponds to i_tx_mii_d[7:0], i_tx_mii_c corresponds to i_tx_mii_d[15:8], and so on.
If the value of a bit is 1, the corresponding data byte is a control byte. If the value of a bit is 0, the corresponding data byte is data.
The Start of Packet byte (0xFB), End of Packet byte (0xFD), Idle bytes (0x07), and error byte (0xFE) are control bytes, but the preamble bytes, Start of Frame (SFD) byte (0xD5), CRC bytes, and payload bytes are data bytes.
While the TX MII valid signal has the value of 0 or the alignment marker insertion bit signal has the value of 1, you must freeze the value of this signal.
|1 bit for each channel||Indicates that the TX MII data signal is valid.
You must assert this signal a fixed number of clock cycles after the IP core raises ready signal, and must deassert this signal the same number of clock cycles after the IP core deasserts the ready signal. The number must be in the range of 1–10 clock cycles.
While you hold the value of this signal at 0, you must freeze the values of both TX MII data and TX MII control bits signals stable.
|1 bit for each channel||Indicates the PCS is ready to receive new data.|
|1 bit for each channel||Alignment marker insertion bit.
The number of valid clock cycles from deassertion of the alignment marker insertion bit signal to reassertion of the alignment marker insertion bit signal is the am_period.For an example that handles this setting for simulation and drives the i_tx_mii_am signal appropriately for simulation, refer to the IP core design example for PCS Only variations. For information about how to generate the IP core design example, refer to the E-Tile Hard IP for Ethernet Design Example User Guide. For information about the sim_mode RTL parameter, refer to the RTL Parameters section of this user guide.
While you hold the value of this signal at 1, you must freeze the values of both TX MII data and TX MII control bits signals.
The figure above shows how to write packets directly to the PCS mode TX interface.
- The packets are written using MII.
- Each byte in i_tx_mii_d has a corresponding bit in i_tx_mii_c that indicates whether the byte is a control byte or a data byte; for example, i_tx_mii_c is the control bit for i_tx_mii_d[15:8].
- i_tx_mii_valid should conform to these conditions:
- Assert the valid signal only when the ready signal is asserted, and deassert only when the ready signal is deasserted.
- The two signals can be spaced by a fixed latency between 1 and 10 cycles.
- When the valid signal deasserts, i_tx_mii_d and i_tx_mii_c must be paused.
- The byte order for the PCS mode TX interface is opposite of the byte order for the MAC client. Bytes flow from right to left; the first byte to be transmitted from the interface is i_tx_mii_d[7:0].
- The bit order for the PCS mode TX interface is the same as the bit order of the MAC client. The first bit to be transmitted from the interface is i_tx_mii_d.
|MII Data||MII Control||Ethernet Packet Byte|
|i_tx_mii_d[7:0]||0xFB||i_tx_mii_c||1||Start of Packet|
The timing of alignment marker insertion is very rigid. Alignment markers cannot be delayed without disrupting the Ethernet link. Use valid cycles to count the alignment markers. When i_tx_mii_valid is low, the alignment marker counters and input must freeze.
The number of cycles for i_tx_mii_am to remain high depends on the rate of the interface:
- Links with RS-FEC: 4 cycles
- Links without RS-FEC: 0 cycle (tie low)
The number of cycles for am period depends on the rate of the interface and whether in simulation or hardware:
- In simulation, it is common to use a reduced am period for both sides of the link to increase lock-time speed. The am period for link with RSFEC enabled is set to 5119.
- In hardware, the am period for link with RSFEC enabled is set to 81916.