Source-Synchronous Signaling With DPA
Figure 3–10. Misaligned Captured Bits
Correct Alignment
0
1
2
3
6
4
7
5
0
6
1
7
2
Incorrect Alignment
3
4
5
The dynamic phase selector and synchronizer align the clock and data
based on the power-up of both communicating devices, and the channel
to channel skew. However, the dynamic phase selector and synchronizer
cannot determine the byte boundary, and the data may need to be
byte-aligned. The dynamic phase aligner’s data realignment circuitry
shifts data bits to correct bit misalignments.
The Stratix GX circuitry contains a data-realignment feature controlled by
the logic array. Stratix GX devices perform data realignment on the
parallel data after the deserialization block. The data realignment can be
performed per channel for more flexibility. The data alignment operation
requires a state machine to recognize a specific pattern. The procedure
requires the bits to be slipped on the data stream to correctly align the
incoming data to the start of the byte boundary.
The DPA uses its realignment circuitry and the global clock for data
realignment. Either a device pin or the logic array asserts the internal
rx_channel_data_alignnode to activate the DPA data-realignment
circuitry. Switching this node from low to high activates the realignment
circuitry and the data being transferred to the logic array is shifted by
one bit. The data realignment block cannot be bypassed. However, if the
rx_channel_data_alignis not turned on (through the altvlds
MegaWizard Plug-In Manager), or when it is not toggled, it only acts as a
register latency.
A state machine and additional logic can monitor the incoming parallel
data and compare it against a known pattern. If the incoming data pattern
does not match the known pattern, you can activate the
rx_channel_data_alignnode again. Repeat this process until the
realigner detects the desired match between the known data pattern and
incoming parallel data pattern.
Altera Corporation
August 2005
3–13
Stratix GX Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
