Receiver Termination; 8B/10B Decoder; Loopback; Table 2-6: Loopback Options - Xilinx RocketIO User Manual

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Receiver Termination

On-chip termination is provided at the receiver, eliminating the need for external
termination. The receiver includes programmable on-chip termination circuitry for 50Ω
(default) or 75Ω impedance.

8B/10B Decoder

An optional 8B/10B decoder is included. A programmable option allows the decoder to be
bypassed. (See
page
Refer to
The decoder uses the same table that is used for Gigabit Ethernet, Fibre Channel and
InfiniBand. In addition to decoding all data and K-characters, the decoder has several extra
features. The decoder separately detects both "disparity errors" and "out-of-band" errors.
A disparity error occurs when a 10-bit character is received that exists within the 8B/10B
table, but has an incorrect disparity. An out-of-band error occurs when a 10-bit character is
received that does not exist within the 8B/10B table. It is possible to obtain an out-of-band
error without having a disparity error. The proper disparity is always computed for both
legal and illegal characters. The current running disparity is available at the RXRUNDISP
signal.
The 8B/10B decoder performs a unique operation if out-of-band data is detected. If out-of-
band data is detected, the decoder signals the error and passes the illegal 10-bits through
and places them on the outputs. This can be used for debugging purposes if desired.
The decoder also signals reception of one of the 12 valid K-characters. In addition, a
programmable comma detect is included. The comma detect signal registers a comma on the
receipt of any comma+, comma–, or both. Since the comma is defined as a 7-bit character, this
includes several out-of-band characters. Another option allows the decoder to detect only
the three defined commas (K28.1, K28.5, and K28.7) as comma+, comma–, or both. In total,
there are six possible options, three for valid commas and three for "any comma".
Note that all bytes (1, 2, or 4) at the RX FPGA interface each have their own individual
8B/10B indicators (K-character, disparity error, out-of-band error, current running
disparity, and comma detect).

Loopback

To facilitate testing without having the need to either apply patterns or measure data at
GHz rates, two programmable loopback features are available.
One option, serial loopback, places the gigabit transceiver into a state where transmit data
is directly fed back to the receiver. An important point to note is that the feedback path is
at the output pads of the transmitter. This tests the entirety of the transmitter and receiver.
The second loopback path is a parallel path that checks the digital circuitry. When the
parallel option is enabled, the serial loopback path is disabled. However, the transmitter
outputs remain active and data is transmitted over a link. If TXINHIBIT is asserted, TXN is
forced to 1 and TXP is forced to 0 until TXINHIBIT is de-asserted.
The two loopback options are shown in

Table 2-6: Loopback Options

20
HDL Code Examples: Transceiver Bypassing of 8B/10B Encoding,
70.) When the 8B/10B decoder is bypassed, the 10-bit character order is:
RXCHARISK[0]
RXRUNDISP[0]
RXDATA[7:0]
Figure 3-11, page
LOOPBACK[1:0]
LOOPBACK[1]
LOOPBACK[0]
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Chapter 2: RocketIO Transceiver Overview
59, for a graphical representation of the received 10-bit character.
Table 2-6.
External serial loopback
Internal parallel loopback
Description
UG024 (v1.5) October 16, 2002
RocketIO™ Transceiver User Guide