The LT transmit block constructs a 4,384-bit frame that contains a frame delimiter, control channel, and link training sequence. It is formatted as follows:
Figure 1. Link Training Frame Structure
It is recommended that the control channel bits not be changed by the link
training algorithm while the transmit state machine is in the process of transmitting
them or they can be received incorrectly, possibly resulting in a DME error. The time
begins when tx_SOF is asserted and ends at least 288 bit
times later, or approximately 30 ns.
Note: Although the coefficient and status contain 128 bit times at
the line rate, the actual signaling rate for these two fields is reduced by a factor
of 8. Therefore, the DME clock rate is one quarter of the line rate.
- Frame Marker
- The frame marker consists of 16 consecutive 1s followed by 16 consecutive 0s. This pattern is not repeated in the remainder of the frame.
- Coefficient and Status
- Because the DME signaling rate for these two fields is reduced by a factor
of 8, each coefficient and status transmission contain 128/8=16 bits each
numbered from 15:0. The following two tables define these bits in the order in
which they are transmitted starting with bit 15 and ending with bit 0.
Table 1. Coefficient and Update Field Bit Definitions Bits Name Description 15:14 Reserved Transmitted as 0, ignored on reception. 13 Preset 1 = Preset coefficients 0 = Normal operation 12 Initialize 1 = Initialize coefficients 0 = Normal operation 11:6 Reserved Transmitted as 0, ignored on reception. 5:4 Coefficient (+1) update - 11 = reserved
- 10 = decrement
- 01 = increment
- 00 = hold
3:2 Coefficient (0) update - 11 = reserved
- 10 = decrement
- 01 = increment
- 00 = hold
1:0 Coefficient (-1) update - 11 = reserved
- 10 = decrement
- 01 = increment
- 00 = hold
Table 2. Status Report Field Bit Definitions Bits Name Description 15 Receiver ready 1 = The local receiver has determined that training is complete and is prepared to receive data.
0 = The local receiver is requesting that training continue.
14:6 Reserved Transmitted as 0, ignored on reception. 5:4 Coefficient (+1) update - 11 = maximum
- 10 = minimum
- 01 = updated
- 00 = not updated
3:2 Coefficient (0) update - 11 = maximum
- 10 = minimum
- 01 = updated
- 00 = not updated
1:0 Coefficient (-1) update - 11 = maximum
- 10 = minimum
- 01 = updated
- 00 = not updated
- Training Sequence
- The training sequence consists of a Pseudo Random Bit Sequence (PRBS) of 4,094 bits followed by two zeros, for a total of 4,096 bits. The PRBS is transmitted at the line rate of 10.3125 or 25.78125 Gbps. The PRBS generator receives an 11-bit seed from an external source. Seed must be non-zero. Subsequent to the initial seed being loaded, the PRBS generator continues to run with no further intervention being required.