
Transmitter
MC92604 Dual Gigabit Ethernet Transceiver Reference Manual, Rev. 1
2-6
Freescale Semiconductor
2.4.1
Transmitting Uncoded Data—8-/4-Bit Modes
The settings for the transmitter control signals when sending uncoded 8-bit or reduced interface 4-bit data
When XMIT_x_ENABLE is low, an IDLE (K28.5) code group of proper running disparity is generated.
The states of the XMIT_x_7–XMIT_x_0, XMIT_x_K, and XMIT_x_ERR signals are ignored. This allows
the link partner’s receiver to maintain alignment when transmission of data is not needed.
When XMIT_x_ENABLE is high, uncoded data is presented in 8-/4-bit bytes to the input register through
the XMIT_x_7–XMIT_x_0 signals. The uncoded data is coded into 10-bit transmission code groups using
an on-chip 8B/10B encoder. 8B/10B coding ensures DC balance across the link and sufficient transition
density to facilitate reliable data and clock recovery. The XMIT_x_7–XMIT_x_0 signals are interpreted
as normal data when the XMIT_x_K signal is low.
The 8B/10B code set includes 12 special control codes. Special control codes may be transmitted by setting
the XMIT_x_K high as indicated in
Table 2-3. There are only 12 valid control code groups; if the data input
is other than the 12 defined values, then an illegal 10-bit code group will be generated and transmitted.
Invalid combinations of XMIT_x_K and XMIT_x_7–XMIT_x_0 will generate invalid 10-bit code groups.
If XMIT_x_ERR is high then the 8B/10B encoder is forced to produce an invalid 10-bit code.
When using the device in a system where word alignment is required, it may be desirable to generate
disparity-style word synchronization events. Also, it may be necessary to generate a disparity-style
synchronization event for compatibility with legacy transceivers. A disparity-style word synchronization
event is generated by setting the transmit data inputs to a 0xAD and asserting XMIT_x_K high for the
appropriate transmitter(s). The transmitter generates 1 of 2 unique 16-code group IDLE (K28.5) sequences
depending on the current running disparity:
I+, I+, I–, I–, I+, I–, I+, I–, I+, I–, I+, I–, I+, I–, I+, I–
or
I–, I–, I+, I+, I–, I+, I–, I+, I–, I+, I–, I+, I–, I+, I–, I+
where I+ stands for K28.5 of positive disparity, and I– stands for K28.5 of negative disparity.
Table 2-3. Transmitter Control States for Uncoded Data (TBIE = Low)
XMIT_x_ENABLE
XMIT_x_ERR
XMIT_x_K
Description
Low
Don’t care
Transmit IDLE (K28.5), ignore XMIT_x_7–XMIT_x_0 data inputs
High
Low
Transmit data present on XMIT_x_7–XMIT_x_0 data inputs
High
Low
High
Transmit control data present on XMIT_x_7–XMIT_x_0 data
inputs
High
Low
High
Transmit disparity-style word synchronization event if
XMIT_x_7–0 data inputs = 0xAD. The transmitter inputs will be
ignored while sending these 16 code groups.
High
Don’t care
Create an invalid 10-bit code group to be transmitted