Altera Corporation
2–37
October 2007
Stratix II GX Device Handbook, Volume 2
Stratix II GX Transceiver Architecture Overview
It is possible for an 8B/10B decoder to decode an invalid control word
encoded into a valid Dx.y code without asserting any code error flags. For
example, depending on the current running disparity, the invalid code
K24.1 (
tx_datain
= 8'h38 +
tx_ctrl
= 1'b1) can be encoded to
10'b0110001100 (0×18C), which is equivalent to a D24.6+ (8'hD8 from the
RD+ column). Altera recommends that you do not send invalid control
words.
Double-Width Mode
In double-width mode, the 8B/10B encoder operates in a cascaded mode.
The least significant byte is transmitted prior to the most significant byte.
Figure 2–30
shows the active 8B/10B encoder blocks in double-width
mode.
Figure 2–30. Active 8B/10B Encoder Blocks in Double-Width Mode
Note to
Figure 2–30
:
(1)
The control signal is
tx_ctrlenable
.
20-Bit Encoding
In double-width mode, the cascaded 8B/10B encoders generate two
10-bit code groups from two 8-bit data and their respective control
identifiers. The 8B/10B encoder forwards the current running disparity
value from the LSByte encoder to the MSByte encoder to calculate the
disparity of the symbol going into the MSByte encoder. The MSByte
encoder’s ending running disparity is then fed back to the LSByte
encoder on the next clock cycle.
8B/10B
Encoder
MSByte
dataout[19..10]
From Byte
Serializer
datain[15..8]
Control Signals[1]
(1)
8B/10B
Encoder
LSByte
dataout[9..0]
datain[7..0]
Control Signals[0]
To Serializer
(1)