Achronix Speedster22i SerDes - Manual (Page 24)

Modes of Operation
Manual Mode:
In manual alignment mode, the symbol alignment will attempt to identify a pre-configured
pattern and lock to the incoming de-serialized data-stream from the output of the PMA or
phase picking block. The alignment operation is triggered by the user logic in the FPGA on
the rising edge of RX_com_det_en. The symbol alignment block then searches for the pre-
configured alignment pattern with or without trailing sequence pattern. Fabric will wait for
the lock status. Once lock to the incoming stream is achieved, the fabric can monitor error
status from the 8b/10b decoder or employ any other mechanism in fabric to identify loss of
lock. The Fabric asserts another rising edge to trigger a new alignment cycle.
Bit Slip Mode:
In bit slip mode, the user logic controls the symbol alignment using the RX_bit_slip_en
signal. Each rising edge of RX_bit_slip_en causes the symbol alignment logic to shift the
word boundary by 1-bit, and symbol alignment will attempt to match the alignment pattern
within the new word boundary. If the word boundary is not matched, the user logic can
again assert RX_bit_slip_en, possibly after waiting for a timeout causing the word boundary
to shift by another bit position. This loop continues until lock is achieved. Once lock to the
incoming stream is achieved, logic in the fabric can monitor error status from 8b/10b decoder
or employ some other mechanism in fabric to identify loss of lock. The bit slip mode supports
all attributes used for manual alignment mode. The maximum number of slips that will cause
a true change in alignment is limited to the data path width.
Automatic Mode:
In automatic alignment mode, the symbol alignment block will automatically determine the
location of the word boundary based on the pre-configured alignment characters. It will also
establish a lock acquired condition based on receiving a pre-con d count of alignment
characters (hysteresis). A loss of lock condition also can be detected by this block based on a
pre-configured count of bad code words (or alignment characters at a different word
boundary). Instead of counting every bad code word, the user can decide to count every ‘n’
bad code word for an incrementing unlock count. Also, the user can use decode/disparity
errors as per clause 36 of IEEE 802.3 to increment and decrement the unlock counter. Support
for Fiber Channel protocol involves synchronization with the 4-symbol wide transmission
word (a special code word K28.5 followed by 3 data code words). In case of Fiber Channel,
any malformed transmission word causes the symbol alignment to go out of lock based on
the un-lock count programmed.
Comma symbols are used for identifying the correct symbol boundary. Section – “Symbols
and Comma Character” introduces comma symbols and discusses on how they are used in
data output from 8b/10b encoder on the TX side of a SerDes. At the receiver end, the
incoming data is scanned for comma symbols. Once the comma symbol is found, the
deserializer resets the word boundary of the received data. The received data is continuously
scanned for the subsequent comma symbols.
24 UG028, July 1, 2014