Activating Comma Alignment; Alignment Status Signals - Xilinx 7 Series User Manual

Chapter 4: Receiver
Figure 4-29
wild-carded comma for a 20-bit internal comma. If ALIGN_COMMA_DOUBLE is TRUE, the
MCOMMA and PCOMMA patterns are combined so that the block searches for two commas in a
row. The number of bits in the comma depends on RX_DATA_WIDTH. Either a 16-bit or a 20-bit
comma alignment mode is possible. A double comma is only detected when the received data has a
PCOMMA defined by ALIGN_PCOMMA_VALUE followed by an MCOMMA defined by
ALIGN_MCOMMA_VALUE with no extra bits in between.
X-Ref Target - Figure 4-29

Activating Comma Alignment

Commas are aligned to the closest boundary providing they are found while comma alignment is
active. RXMCOMMAALIGNEN is driven High to align on the MCOMMA pattern.
RXPCOMMAALIGNEN is driven High to activate alignment on the PCOMMA pattern. Both
enable ports are driven to align to either pattern. When ALIGN_COMMA_DOUBLE is TRUE, both
enable ports must always be driven to the same value.

Alignment Status Signals

While MCOMMA or PCOMMA alignment is active, any matching comma pattern causes the block
to realign to the closest boundary. After successful alignment, the block holds
RXBYTEISALIGNED High. At this time, RXMCOMMAALIGNEN and
RXPCOMMAALIGNEN can be driven Low to turn off alignment and keep the current alignment
position. RXPCOMMAALIGNEN must be TRUE for PCOMMAs to cause RXBYTEISALIGNED
to go High. Similarly, RXMCOMMAALIGNEN must be TRUE for MCOMMAs to cause
RXBYTEISALIGNED to go High. Commas can arrive while RXBYTEISALIGNED is High. If the
commas arrive aligned to boundaries, there is no change. If the commas arrive out of position, the
block deasserts RXBYTEISALIGNED until the commas are aligned again. If alignment is still
activated for the comma that arrives, the block automatically aligns the new comma to the closest
boundary and drives RXBYTEREALIGN High for one RXUSRCLK2 cycle.
In applications that operate at a line rate greater than 5 Gb/s and have excessive noise in the system,
the byte align block might falsely align to a wrong byte boundary and falsely assert the
RXBYTEISALIGNED signal when no valid data is present. In such applications, a system level
check should be in place for checking the validity of the RXBYTEISALIGNED indicator and data.
In systems that use the OOB (RX Out-of-Band signaling) block e.g., PCIe, SATA, after locking to a
valid byte boundary and asserting the RXBYTEISALIGNED signal, the byte align block might
occasionally de-assert the RXBYTEISALIGNED signal even when there is no change in the byte
162
Send Feedback
shows how a comma is combined with ALIGN_COMMA_ENABLE to make a
ALIGN_MCOMMA_VALUE
and
ALIGN_PCOMMA_VALUE
(ALIGN_COMMA_DOUBLE = TRUE)
001010 0 001 001010 0 0 10
001111 1 111 001111 1 1 11
00111 1 1 1 1 1
ALIGN_COMMA_ENABLE
Figure 4-29: Extended Comma Pattern Masking
www.xilinx.com
Pattern Required for
Comma Detection
(x = don't care)
x x1 0 1 00 0 0 1
7 Series FPGAs GTP Transceivers User Guide
UG482 (v1.9) December 19, 2016
x x1 0 10 0 0 1 0
UG482_c4_18_111011