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Figure 8 shows the timing waveform for read data and strobe delay determination. The
waveforms on the left show a case where the DQS is delayed due to BUFIO and clocking
resource, and the ISERDES outputs do not match the expected data pattern. The waveforms
on the right show a case where the DQS and DQ are delayed until the ISERDES outputs match
the expected data pattern. The lower end of the frequency range useful in this design is limited
by the number of available taps in the IDELAY block, the PCB trace delay, and the CAS latency
of the memory device.
Figure 8: Read Data and Strobe Delay
CLKdiv_0
CLKdiv_90
CLKfast_90
CLKfast_0
DQS @ FPGA
DQ @ FPGA
Parallel Data @ ISERDES
Outputs Q4, Q3, Q2, Q1
DQS @ ISERDES
delayed by BUFIO
and clocking resource
DQ
D0 D1 D2 D3
D0
D0
D1 D2
D2
D3
D1 D3
D0 D2
D1 D3
D0 D2
D1
D2,D3,D0,D1
D3
X721_08_112905
No Match
Incorrect Data
Sequence
DQS @ FPGA
DQ @ FPGA
Parallel Data @ ISERDES
Outputs Q4, Q3, Q2, Q1
DQS Delayed by Calibration
Delay @ ISERDES
DQ Delayed by Calibration Delay
D0 D1 D2 D3
D0
D0
D1 D2
D2
D3
D1 D3
D0 D2
D1 D3
D0 D2
D1
D0,D1,D2,D3
D3
Correct Data
Sequence
DQ Captured in DQS Domain
D0
D1
CLKfast_90
Domain
Input to Q2 Reg
Input to Q1 Reg
Input to Q3 Reg
Input to Q4 Reg
March 2006 Memory Interfaces Solution Guide 63