1.8.4 LPDDR and DDR2 Design

The designer must be familiar with the specification and the basic electrical operation of the LPDDR/DDR2 interface. Data bus, data strobe, and data mask (byte enable) signals are point-to-point, whereas all other address, control, and clock signals are not point-to-point. The following figures show the connectivity of the SmartFusion2/IGLOO2 LPDDR interface and a 32-bit DDR2 interface respectively.

Figure 1-16. LPDDR Interface
Note:
  1. Impedance calibration is optional for LPDDR operating in LVCMOS mode and is required for LPDDR1 operating in SSTL18 mode.

  2. For a 4-bit or 8-bit DRAM, all DQ pins are interchangeable. All 4-bit and 8-bit DQ pins are interchangeable in LPDDR, DDR2, and DDR3 memories. For a 16-bit DRAM, DQ0 through DQ7 are interchangeable. Also, DQ8 through DQ15 are interchangeable. However, DQ0-7 pins or signals must not be interchanged with the DQ8-15 pins or signals.

  3. Short ECC_TMATCH_ OUT and ECC_TMATCH_ IN when using ECC bits.

Figure 1-17. DDR2 Interface

With short traces, the address, control, and command signals might not require both parallel (RT) and series (RS) termination. In a worst-case scenario, a small series resistor (RS) of about 10 Ω or less is required. This series termination is not used for impedance matching, but for dampening the signals.

Note:
  1. To get length matching, short the TMATCH_OUT to TMATCH_IN with the shortest loop.
  2. Short ECC_TMATCH_ OUT and ECC_TMATCH_ IN when using ECC bits.