Mobile DDR

Mobile DDR (also known as mDDR, Low Power DDR, or LPDDR) is type of double data rate synchronous DRAM for mobile computers.


Original LPDDR

The original low-power DDR (sometimes, in hindsight, called LPDDR1) is a slightly modified form of DDR SDRAM, with several changes to reduce overall power consumption.

Most significant, the supply voltage is reduced from 2.5 to 1.8 V. Additional savings come from temperature-compensated refresh (DRAM requires refresh less often at low temperatures), partial array self refresh, and a "deep power down" mode which sacrifices all memory contents. Additionally, chips are smaller, using less board space than their non-mobile equivalents. Samsung and Micron are two of the main providers of this technology, and is used in tablet computing devices such as the Apple iPad, Samsung Galaxy Tab and Motorola Droid X[1].


A new JEDEC standard JESD209-2E defines a more dramatically revised low-power DDR interface. It is not compatible with either DDR1 or DDR2 SDRAM, but can accommodate either:

  • LPDDR2-S2: 2n prefetch memory (like DDR1),
  • LPDDR2-S4: 4n prefetch memory (like DDR2), or
  • LPDDR2-N: Non-volatile (NAND flash) memory.

Low-power states are similar to basic LPDDR, with some additional partial array refresh options.

Timing parameters are specified for LPDDR-200 to LPDDR-1066 (clock frequencies of 100 to 533 MHz).

Working at 1.2 V, LPDDR2 multiplexes the control and address lines onto a 10-bit double data rate CA bus. The commands are similar to those of normal SDRAM, except for the reassignment of the precharge and burst terminate opcodes:

LPDDR2 command encoding[2]
CA3 CA4 CA5 CA6 CA7 CA8 CA9 Operation
H H L H H Precharge all banks
H H L H L BA2 BA1 BA0 Precharge one bank
H H L H A30 A31 A32 BA2 BA1 BA0 Preactive
(LPDDR2-N only)
A20 A21 A22 A23 A24 A25 A26 A27 A28 A29
H H L L Burst terminate
H L H reserved C1 C2 BA2 BA1 BA0 Read
AP C3 C4 C5 C6 C7 C8 C9 C10 C11
H L L reserved C1 C2 BA2 BA1 BA0 Write
AP C3 C4 C5 C6 C7 C8 C9 C10 C11
L H R8 R9 R10 R11 R12 BA2 BA1 BA0 Activate
(R0–14=Row address)
R0 R1 R2 R3 R4 R5 R6 R7 R13 R14
L H A15 A16 A17 A18 A19 BA2 BA1 BA0 Activate
(LPDDR2-N only)
A5 A6 A7 A8 A9 A10 A11 A12 A13 A14
L L H H Refresh all banks
(LPDDR2-Sx only)
L L H L Refresh one bank
(Round-robin addressing)
L L L H MA0 MA1 MA2 MA3 MA4 MA5 Mode register read
L L L L MA0 MA1 MA2 MA3 MA4 MA5 Mode register write

Column address bit C0 is never transferred, and is assumed to be zero. Burst transfers thus always begin at even addresses.

LPDDR2 also has an active-low chip select (when high, everything is a NOP) and clock enable CKE signal, which operate like SDRAM. Also like SDRAM, the command sent on the cycle that CKE is first dropped selects the power-down state:

  • If the chip is active, it freezes in place.
  • If the command is a NOP (CS low or CA0–2 = HHH), the chip idles.
  • If the command is a refresh command (CA0–2 = LLH), the chip enters the self-refresh state.
  • If the command is a burst terminate (CA0–2 = HHL), the chip enters the deep power-down state. (A full reset sequence is required when leaving.)

The mode registers have been greatly expanded compared to conventional SDRAM, with an 8-bit address space, and the ability to read them back. Although smaller than a serial presence detect EEPROM, enough information is included to obviate the need for one.

S2 devices smaller than 4 Gbit, and S4 devices smaller than 1 Gbit have only 4 banks. They ignore the BA2 signal, and do not support per-bank refresh.

Non-volatile memory devices do not use the refresh commands, and reassign the precharge command to transfer address bits A20 and up. The low-order bits (A19 and down) are transferred by a following Activate command. This transfers the selected row from the memory array to one of 4 or 8 (selected by the BA bits) row data buffers, where they can be read by a Read command. Unlike DRAM, the bank address bits are not part of the memory address; any address can be transferred to any row data buffer. A row data buffer may be from 32 to 4096 bytes long, depending on the type of memory. Rows larger than 32 bytes ignore some of the low-order address bits in the Activate command. Rows smaller than 4096 bytes ignore some of the high-order address bits in the Read command.

Non-volatile memory does not support the Write command to row data buffers. Rather, a series of control registers in a special address region support Read and Write commands, which can be used to erase and program the memory array.


External links