External Memory Interface Handbook Volume 2: Design Guidelines

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ID 683385
Date 5/08/2017
Public
Document Table of Contents

1.1.4. DDR, DDR2, DDR3, and DDR4 SDRAM Data, Data Strobes, DM/DBI, and Optional ECC Signals

DDR SDRAM uses bidirectional single-ended data strobe (DQS); DDR3 and DDR4 SDRAM use bidirectional differential data strobes. The DQSn pins in DDR2 SDRAM devices are optional but recommended for DDR2 SDRAM designs operating at more than 333 MHz. Differential DQS operation enables improved system timing due to reduced crosstalk and less simultaneous switching noise on the strobe output drivers. The DQ pins are also bidirectional.

Regardless of interface width, DDR SDRAM always operates in ×8 mode DQS groups. DQ pins in DDR2, DDR3, and DDR4 SDRAM interfaces can operate in either ×4 or ×8 mode DQS groups, depending on your chosen memory device or DIMM, regardless of interface width. The ×4 and ×8 configurations use one pair of bidirectional data strobe signals, DQS and DQSn, to capture input data. However, two pairs of data strobes, UDQS and UDQS# (upper byte) and LDQS and LDQS# (lower byte), are required by the ×16 configuration devices. A group of DQ pins must remain associated with its respective DQS and DQSn pins.

The DQ signals are edge-aligned with the DQS signal during a read from the memory and are center-aligned with the DQS signal during a write to the memory. The memory controller shifts the DQ signals by –90 degrees during a write operation to center align the DQ and DQS signals. The PHY IP delays the DQS signal during a read, so that the DQ and DQS signals are center aligned at the capture register. Intel® devices use a phase-locked loop (PLL) to center-align the DQS signal with respect to the DQ signals during writes and Intel® devices use dedicated DQS phase-shift circuitry to shift the incoming DQS signal during reads. The following figure shows an example where the DQS signal is shifted by 90 degrees for a read from the DDR2 SDRAM.

Figure 1. Edge-aligned DQ and DQS Relationship During a DDR2 SDRAM Read in Burst-of-Four Mode


The following figure shows an example of the relationship between the data and data strobe during a burst-of-four write.

Figure 2. DQ and DQS Relationship During a DDR2 SDRAM Write in Burst-of-Four Mode


The memory device's setup (tDS) and hold times (tDH) for the DQ and DM pins during writes are relative to the edges of DQS write signals and not the CK or CK# clock. Setup and hold requirements are not necessarily balanced inDDR2 and DDR3 SDRAM, unlike in DDR SDRAM devices.

The DQS signal is generated on the positive edge of the system clock to meet the tDQSS requirement. DQ and DM signals use a clock shifted –90 degrees from the system clock, so that the DQS edges are centered on the DQ or DM signals when they arrive at the DDR2 SDRAM. The DQS, DQ, and DM board trace lengths need to be tightly matched (within 20 ps).

The SDRAM uses the DM pins during a write operation. Driving the DM pins low shows that the write is valid. The memory masks the DQ signals if the DM pins are driven high. To generate the DM signal, Intel® recommends that you use the spare DQ pin within the same DQS group as the respective data, to minimize skew.

The DM signal's timing requirements at the SDRAM input are identical to those for DQ data. The DDR registers, clocked by the –90 degree shifted clock, create the DM signals.

DDR4 supports DM similarly to other SDRAM, except that in DDR4 DM is active LOW and bidirectional, because it supports Data Bus Inversion (DBI) through the same pin. DM is multiplexed with DBI by a Mode Register setting whereby only one function can be enabled at a time. DBI is an input/output identifying whether to store/output the true or inverted data. When enabled, if DBI is LOW, during a write operation the data is inverted and stored inside the DDR4 SDRAM; during a read operation, the data is inverted and output. The data is not inverted if DBI is HIGH. For Arria 10, the DBI (for DDR4) and the DM (for DDR3) pins in each DQS group must be paired with a DQ pin for proper operation.

Some SDRAM modules support error correction coding (ECC) to allow the controller to detect and automatically correct error in data transmission. The 72-bit SDRAM modules contain eight extra data pins in addition to 64 data pins. The eight extra ECC pins should be connected to a single DQS or DQ group on the FPGA.

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