Altera Stratix GX Transceiver - Manual (Page 54)

3–8 Altera Corporation
Stratix GX Transceiver User Guide January 2005
Basic Mode Receiver Architecture
deasserted and the A1 pattern is present on the rx_word_align_out
port. At time n+6, the A2 pattern is present on the rx_word_align_out
port. The word aligner then asserts the rx_patterndetect signal for
one clock cycle to flag the detection of the comma on the current word
boundary.
Manual Bit-Slipping Alignment Mode
You can also achieve word alignment by enabling the manual bit-slip
option. With this option enabled, the transceiver has the ability to shift the
word boundary one-bit every parallel clock cycle. Bits are shifted from the
MSB to LSB direction. Shifting occurs every time the bit-slipping circuitry
detects a rising edge of the rx_bitslip[] signal. Each time a bit is
slipped, the bit that arrived at the receiver earlier is skipped. When the
word boundary matches what is specified as the comma, the
rx_patterndetect[]
signal is asserted for one clock cycle. For best
results, implement the logic in the device logic array to control the bit-slip
circuitry.
This scheme is useful if the comma changes dynamically when the
Stratix GX device is in user mode. Because the controller is implemented
in the logic array, a custom controller can be built to dynamically change
the comma without needing to reprogram the Stratix GX device.
Figure 3–7 shows an example of how the word aligner signals interact in
the manual bitslip alignment mode. For this example, 8'b00111100 is
specified as the comma, and an 8'b11110000 value is held at the rx_in
port. Every rising edge on the rx_bitslip port causes the
rx_word_align_out
data to shift a bit from the MSB to the LSB. This
shifting 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 is held
low, because the specified comma does not exist in the current word
boundary. The rx_bitslip is disabled at time n+3 and re-enabled at
time n+4. The output of the rx_word_align_out now matches the
specified comma, so the rx_patterndetect is asserted for one clock
cycle. At time n+5 the rx_patterndetect is still asserted since the
comma still exists in the current word boundary. Finally, at time n+6 the
rx_word_align_out
boundary is shifted again, and the
rx_patterndetect signal is deasserted to signify that the word
boundary does not contain the comma.