HC4GX15 Datasheet, PDF (311/668 Pages) Altera Corporation – HardCopy IV Device Handbook

Chapter 1: HardCopy IV GX Transceiver Architecture
1–47
Transmitter Channel Datapath
K28.5 at the low byte position in time n + 4 should be encoded with a positive
disparity. Because tx_forcedisp is high at time n + 4, the low signal level of
tx_dispval is used to convert the lower byte K28.5 to be encoded as a positive
disparity code word. As the upper bit of tx_forcedisp is low at n + 4, the high byte
K28.5 takes the current running disparity from the low byte.
Figure 1–34. 8B/10B Encoder Force Current Running Disparity in Double-Width Mode
clock
tx_datain[15:0]
tx_ctrlenable[1:0]
n
BC BC
n+2
BC BC
11
n+4
BC BC
BC BC
tx_forcedisp[1:0]
tx_dispval[1:0]
Current Running
Disparity
tx_dataout[19:0]
00
01
00
RD- RD ⁺ RD-
00
RD+
RD+ RD-
RD+ RD-
17C 283 17C 283 283 17C 283 17C
Transmitter Polarity Inversion
The positive and negative signals of a serial differential link might accidentally be
swapped during board layout. Solutions like a board re-spin or major updates to the
logic in the core fabric can be expensive. The transmitter polarity inversion feature is
provided to correct this situation. An optional tx_invpolarity port is available in
all functional modes except (OIF) CEI PHY to dynamically enable the transmitter
polarity inversion feature.
In single-width mode, a high value on the tx_invpolarity port inverts the polarity
of every bit of the 8-bit or 10-bit input data word to the serializer in the transmitter
datapath. In double-width mode, a high value on the tx_invpolarity port inverts
the polarity of every bit of the 16-bit or 20-bit input data word to the serializer in the
transmitter datapath. Because inverting the polarity of each bit has the same effect as
swapping the positive and negative signals of the differential link, correct data is seen
by the receiver. tx_invpolarity is a dynamic signal and might cause initial
disparity errors at the receiver of an 8B/10B encoded link. The downstream system
must be able to tolerate these disparity errors.
© June 2009 Altera Corporation
HardCopy IV Device Handbook Volume 3