External Memory Interface Handbook Volume 3: Reference Material

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ID 683841
Date 7/24/2019
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1.17.5. Stage 2: Write Calibration Part One

The objectives of the write calibration stage are to align DQS to the memory clock at each memory device, and to compensate for address, command, and memory clock skew at each memory device. This stage is important because the address, command, and clock signals for each memory component arrive at different times.
Note: This stage applies only to DDR2, DDR3, LPDDR2, and RLDRAM II protocols; it does not apply to the QDR II and QDR II+ protocols.

Memory clock signals and DQ/DM and DQS signals have specific relationships mandated by the memory device. The PHY must ensure that these relationships are met by skewing DQ/DM and DQS signals. The relationships between DQ/DM and DQS and memory clock signals must meet the tDQSS, tDSS, and tDSH timing constraints.

The sequencer calibrates the write data path using a variety of random burst patterns to compensate for the jitter on the output data path. Simple write patterns are insufficient to ensure a reliable write operation because they might cause imprecise DQS-to-CK alignments, depending on the actual capture circuitry on a memory device. The write patterns in the write leveling stage have a burst length of 8, and are generated by a linear feedback shift register in the form of a pseudo-random binary sequence.

The write data path architecture is the same for DQ, DM, and DQS pins. The following figure illustrates the write data path for a DQ signal. The phase coming out of the Output Phase Alignment block can be set to different values to center-align DQS with respect to DQ, and it is the same for data, OE, and OCT of a given output.

Figure 29. Write Data Path


In write leveling, the sequencer performs write operations with different delay and phase settings, followed by a read. The sequencer can implement any phase shift between 0° and 720° (depending on device and configuration). The sequencer uses the Output Phase Alignment for coarse delays and T9 and T10 for fine delays; T9 has 15 taps of 50 ps each, and T10 has 7 taps of 50 ps each.

The DQS signal phase is held at +90° with respect to DQ signal phase (Stratix IV example).

Note: Coarse delays are called phases, and fine delays are called delays; phases are process, voltage, and temperature (PVT) compensated, delays are not (depending on family).

For 28 nm devices:

  • I/O delay chains are not PVT compensated.
  • DQS input delay chain is PVT compensated.
  • Leveling delay chains are PVT compensated (does not apply to Arria V or Cyclone V devices).
  • T11 delay chain for postamble gating has PVT and nonPVT compensated modes, but the PVT compensated mode is not used.
Note: Delay and phase values used in this section are examples, for illustrative purposes. Your exact values may vary depending on device and configuration.

The sequencer writes and reads back several burst-length-8 patterns. Because the sequencer has not performed per-bit deskew on the write data path, not all bits are expected to pass the write test. However, for write calibration to succeed, at least one bit per group must pass the write test. The test begins by shifting the DQ/DQS phase until the first write operation completes successfully. The DQ/DQS signals are then delayed to the left by D5 and D6 to find the left edge for that working phase. Then DQ/DQS phase continues the shift to find the last working phase. For the last working phase, DQ/DQS is delayed in 50 ps steps to find the right edge of the last working phase.

The sequencer sweeps through all possible phase and delay settings for each DQ group where the data read back is correct, to define a window within which the PHY can reliably perform write operations. The sequencer picks the closest value to the center of that window as the phase/delay setting for the write data path.

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