2–80
Altera Corporation
Stratix II GX Device Handbook, Volume 2
October 2007
Receiver Modules
The byte boundary is locked after the first alignment pattern is detected
and after the rising edge of the
rx_enapatternalign
port. If the byte
boundary changes, the
rx_enapatternalign
port is deasserted and
reasserted to enable the alignment circuit to search for and align to the
next available alignment pattern.
On the rising edge of the
rx_enapatternalign
port, the word aligner
locks onto the first alignment pattern detected. In this scenario,
rx_patterndetect
is asserted to signify that the alignment pattern has
been aligned. The
rx_syncstatus
signal is also asserted for one clock
cycle to signify that the word boundary has been synchronized. After the
word boundary is locked, whether or not
rx_enapatternalign
is held
high or low, the
rx_syncstatus
signal asserts for one clock cycle
whenever the alignment pattern is detected across a different byte
boundary. The
rx_syncstatus
signal operates in this
resynchronization state until a rising edge is detected on the
rx_enapatternalign
port.
Manual Bit-Slip Alignment Mode
You can also achieve word alignment by enabling the manual bit-slip
option in the MegaWizard. With this option enabled, the transceiver shifts
the word boundary MSB to LSB one bit every parallel clock cycle. The
transceiver shifts the word boundary every time the bit-slipping circuitry
detects a rising edge of the
rx_bitslip
signal. At each rising edge of the
rx_bitslip
signal, the word boundary slips one bit. The bit that arrives
at the receiver first is skipped. When the word boundary matches the
alignment pattern you specified in the MegaWizard, the
rx_patterndetect
signal is asserted for one clock cycle. You must
implement the logic in the PLD logic array to control the bit-slip circuitry.
The bit slipper is useful if the alignment pattern changes dynamically
when the Stratix II GX device is in user mode. You can implement the
controller in the logic array, so you can build a custom controller to
dynamically change the alignment pattern without needing to reprogram
the Stratix II GX device.
Figure 2–63
shows an example of how the word aligner signals interact in
the manual bit-slip alignment mode. For this example, 8’b00111100 is
specified as the alignment pattern and an 8’b11110000 value is held at the
rx_datain
port.
Every rising edge on the
rx_bitslip
port causes the
rx_dataout
data
to shift one bit from the MSB to the LSB by default. This is shown at time
n
+ 2 where the 8’b11110000 data is shifted to a value of 8’b01111000. At
this state the
rx_patterndetect
signal is held low because the
specified alignment pattern does not exist in the current word boundary.