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Xilinx XAPP721 manual

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Table of contents for the manual

  • Page 1

    © 2005 – 2006 Xilinx, Inc. All rights reserved. XI LINX, the Xilinx logo , and other de signated br ands included herein are tradem a rks of Xilinx, Inc. All other trademarks are the property of their respectiv e owners . Summary This application note describes a data capture techniq ue fo r a high-perf ormance DDR2 SDRAM interf ace. This te chn[...]

  • Page 2

    Write Datapath R Write Datapath The write datapath uses the built-in OSER DES av ailable in e ver y Vir tex-4 I/O . The OSERDES transmits the data (DQ) and strobe (DQS) signals. The memory specification r equires DQS to be transmitted cent er-aligned with D Q. The strobe ( DQS) forw arded to t he memor y is 180 degre es out of phase with CLKfa st_0[...]

  • Page 3

    Write Datapath R Figure 3: Write Data T ransmitted Using OSERDES Figure 4: Write Str obe (DQS) and Data (DQ) Timing f or a Write Latency of Four D1 D2 D3 D4 CLKDIV CLK CLKdiv_90 CLKfast_90 OSERDES DQ IOB ChipSync TM Circuit Write Data W ords 0-3 X721_03_080305 CLKf ast_0 Clock F orwarded to Memor y De vice Command WRITE IDLE D0 D1 D2 D3 Control (CS[...]

  • Page 4

    Write Datapath R Write Timing Analysis Ta b l e 1 shows the write timing analys is f or an interface at 333 MHz (667 Mb/s). Ta b l e 1 : Write Timing Analysis at 333 MHz Uncertainty Parameter s V alue Uncer tainties before DQS Uncer tain ties after DQS Meaning T CLOCK 3000 Clock period. T MEMOR Y_DLL_DUTY_CYCLE_DIST 150 150 150 Duty-cycle distor ti[...]

  • Page 5

    Write Datapath R Controller to Writ e Datapath Interface Ta b l e 2 lists the signals required f rom the controller to th e write datapath. Ta b l e 2 : Controller to Write Datapath Signals Signal Name Signal Width Signal Description Notes ctrl_WrEn 1 Output from the con troller to the write datapath . Write DQS and DQ generation begins when this s[...]

  • Page 6

    Write Datapath R Figure 5: Write DQ Generation with a Write Latenc y of 4 and a Bur st Length of 4 Figure 6: Write DQS Generation for a Write La tenc y of 4 and a Burst Length of 4 CLKdiv_0 CLKdiv_90 CLKfast_90 Clock F orwarded to Memory Device Command WRITE IDLE D0 D1 D2 D3 Control (CS_L) Strobe (DQS) ctrl_WrEn ctrl_wr_disable OSERDES Inputs D1, D[...]

  • Page 7

    Read Datapath R Read Datapath The read datapath co mprises the read data capture and recaptur e stages . Both stages are implemented in the built-in ISERDES a vailable in e very Vir te x-4 I/O . The IS ERDES has three clock inputs: CLK, OCLK, and CLKDIV . The read data is captured in t he CLK (DQS) domain, recaptured in the OCLK (F PGA f ast cloc k[...]

  • Page 8

    Read Datapath R Ta b l e 3 shows the read timing analysis at 333 MHz re quired to deter mine the dela y required on DQ bits f or center ing DQS in the data v alid window . P er Bit Deskew Data Capture T echnique T o ensure reliable data capture in the OCLK and CLKDIV domains in the ISERDES , a training sequence is required after memory initializati[...]

  • Page 9

    Read Datapath R Figure 8 shows the timing w avef orm f or read data and strobe dela y deter mination. The wa vef or ms on the left show a case where the DQS is dela yed due to BUFIO and clo cking resource, and the ISE RDES outputs do not match the expected data patter n. Th e wa vef or ms on the right show a case where the DQS and DQ are delay ed u[...]

  • Page 10

    Read Datapath R Controller to Read Datapath Interface Ta b l e 4 lists the control signals between the contro ller and the read datapath. Ta b l e 4 : Signals between Contr oller and Read Datapath Signal Name Signal Width Signal Description Notes ctrl _Dummyread_Star t 1 Output fro m the controlle r to the read data path. When th is signal is asser[...]

  • Page 11

    Reference Design R The ctrl_RdEn signal is requir ed to validate read data because the DDR2 SDRAM de vices do not provide a r ead v alid or read-enab le signal along with read data. The controlle r generates this read-enable signal based on the CAS latency and the b urst length. This read-enable signal is input to an SRL16 (LUT -based shift registe[...]

  • Page 12

    Reference Design Utilization R Reference Design Utilization Ta b l e 5 lists the resource utilization f or a 64-bit interf ace including the physical la yer , the controller , the user interf ace, and a synthesizab le test bench. Conc lusion The data capture technique e xplained in th is application note using ISERDES provides a good margin f or hi[...]