L- and H-Tile Transceiver PHY User Guide

ID 683621
Date 1/30/2024
Document Table of Contents 8B/10B Encoding for GbE, GbE with IEEE 1588v2

The 8B/10B encoder clocks 8-bit data and 1-bit control identifiers from the transmitter phase compensation FIFO and generates 10-bit encoded data. The 10-bit encoded data is sent to the PMA.

The IEEE 802.3 specification requires GbE to transmit Idle ordered sets (/I/) continuously and repetitively whenever the gigabit media-independent interface (GMII) is Idle. This transmission ensures that the receiver maintains bit and word synchronization whenever there is no active data to be transmitted.

For the GbE protocol, the transmitter replaces any /Dx.y/ following a /K28.5/ comma with either a /D5.6/ (/I1/ ordered set) or a /D16.2/ (/I2/ ordered set), depending on the current running disparity. The exception is when the data following the /K28.5/ is /D21.5/ (/C1/ ordered set) or /D2.2/ (/C2/) ordered set. If the running disparity before the /K28.5/ is positive, an /I1/ ordered set is generated. If the running disparity is negative, a /I2/ ordered set is generated. The disparity at the end of a /I1/ is the opposite of that at the beginning of the /I1/. The disparity at the end of a /I2/ is the same as the beginning running disparity immediately preceding transmission of the Idle code. This sequence ensures a negative running disparity at the end of an Idle ordered set. A /Kx.y/ following a /K28.5/ does not get replaced.

Note: /D14.3/, /D24.0/, and /D15.8/ are replaced by /D5.6/ or /D16.2/ (for I1 and I2 ordered sets). D21.5 (/C1/) is not replaced.
Figure 61.  Idle Ordered-Set Generation Example

Reset Condition for 8B/10B Encoder in GbE, GbE with IEEE 1588v2

After deassertion of tx_digitalreset, the transmitters automatically transmit at least three /K28.5/ comma code groups before transmitting user data on the tx_parallel_data port. This transmission could affect the synchronization state machine behavior at the receiver.

Depending on when you start transmitting the synchronization sequence, there could be an even or odd number of /Dx.y/ code groups transmitted between the last of the three automatically sent /K28.5/ code groups and the first /K28.5/ code group of the synchronization sequence. If there is an even number of /Dx.y/code groups received between these two /K28.5/ code groups, the first /K28.5/ code group of the synchronization sequence begins at an odd code group boundary. The synchronization state machine treats this as an error condition and goes into the loss of synchronization state.

Figure 62. Reset Condition