Double data rate (ddr)

Double Data Rate (DDR) SDRAM Specification 64Mb through 1Gb DDR SDRAMs with X4/X8/X16 data interfaces.

FEATURES Double--data--rate architecture; two data transfers per clock cycle Bidirectional, data strobe (DQS) is transmitted/received with data, to be used in capturing data at the receiver DQS is edge--aligned with data for READs ; center--aligned with data for WRITEs Differential clock inputs (CK and CK#)

DLL aligns DQ and DQS transitions with CK transitions Commands entered on each positive CK edge; data and data mask referenced to both edges of DQS Four internal banks for concurrent operation Data mask (DM) for write data Burst lengths: 2, 4, or 8 CAS Latency: 2 or 2.5, DDR400 also includes CL = 3

AUTO PRECHARGE option for each burst access Auto Refresh and Self Refresh Modes

BGA Device Address Assignment and Package Table

FUNCTIONAL BLOCK DIAGRAM OF DDR SDRAM

INITIALIZATION Power sequence DESELECT or NOP PRECHARGE ALL MODE REGISTER SET for Extended Mode Register to enable the DLL. MODE REGISTER SET for Mode Register to reset the DLL, and to program the operating parameters. PRECHARGE ALL Two AUTO refresh cycles

MODE REGISTER SET command for the Mode Register, with the reset DLL bit deactivated (i.e., to program operating parameters without resetting the DLL) must be performed. Following these cycles, the DDR SDRAM is ready for normal operation.

INITIALIZE AND MODE REGISTER SETS

MODE REGISTER Define the specific mode of operation 1.Selection of a burst length 2.Burst type 3.CAS latency 4.Operating mode

Burst Length The burst length determines the maximum number of column locations that can be accessed for a given READ or WRITE command.

Burst Type Accesses within a given burst may be programmed to be either sequential or interleaved; this is referred to as the burst type.

Read Latency The READ latency is the delay, in clock cycles, between the registration of a READ command and the availability of the first piece of output data.

EXTENDED MODE REGISTER Control DLL enable/disable. Output drive strength selection (optional).

DESELECT The DESELECT function (CS = High) prevents new commands from being executed by the DDR SDRAM.

NO OPERATION (NOP) This prevents unwanted commands from being registered during idle or wait states.

MODE REGISTER SET The MODE REGISTER SET command can only be issued when all banks are idle and no bursts are in progress, and a subsequent executable command cannot be issued until tMRD is met.

ACTIVE The ACTIVE command is used to open (or activate) a row in a particular bank for a subsequent access. This row remains active (or open) for accesses until a precharge (or READ or WRITE with AUTOPRECHARGE) is issued to that bank.

READ The READ command is used to initiate a burst read access to an active row. The value on input A10 determines whether or not auto precharge is used.

WRITE The WRITE command is used to initiate a burst write access to an active row. The value on input A10 determines whether or not auto precharge is used. Input data appearing on the DQs is written to the memory array subject to the DM input logic level appearing coincident with the data.

BURST TERMINATE The BURST TERMINATE command is used to truncate read bursts (with autoprecharge disabled).

PRECHARGE The PRECHARGE command is used to deactivate the open row in a particular bank or the open row in all banks. Input A10 determines whether one or all banks are to be precharged. A PRECHARGE command will be treated as a NOP if there is no open row in that bank, or if the previously open row is already in the process of precharging.

AUTO PRECHARGE AUTO PRECHARGE is a feature which performs the same individual--bank precharge function described above, but without requiring an explicit command. This is accomplished by using A10 to enable AUTO PRECHARGE in conjunction with a specific READ or WRITE command.

AUTO PRECHARGE ensures that the precharge is initiated at the earliest valid stage within a burst. This is determined as if an explicit PRECHARGE command was issued at the earliest possible time.

REFRESH REQUIREMENTS DDR SDRAMs require a refresh of all rows in any rolling 64 ms interval. Dividing the number of device rows into the rolling 64ms interval defines the average refresh interval, tREFI, which is a guideline to controllers for distributed refresh timing. For example, a 256Mb DDR SDRAM has 8192 rows resulting in a tREFI of 7.8 μ s.

To avoid excessive interruptions to the memory controller, higher density DDR SDRAMs maintain the 7.8 μ s average refresh time and perform multiple internal refresh bursts.

AUTO REFRESH AUTOREFRESH is used during normal operation of the DDR SDRAM The refresh addressing is generated by the internal refresh controller. This makes the address bits ”Don’t Care” during an AUTO REFRESH command. The DDR SDRAM requires AUTO REFRESH cycles at an average periodic interval of tREFI (maximum).

A maximum of eight AUTO REFRESH commands can be posted to any given DDRSDRAM,and the maximum absolute interval between any AUTO REFRESH command and the next AUTO REFRESH command is 8 * tREFI.

SELF REFRESH When in the self refresh mode, the DDR SDRAM retains data without external clocking. The SELF REFRESH command is initiated like an AUTO REFRESH command except CKE is disabled (LOW). The DLL is automatically disabled upon entering SELF REFRESH, and is automatically enabled upon exiting SELF REFRESH.

Any time the DLL is enabled a DLL Reset must follow and 200 clock cycles should occur before a READ command can be issued. Input signals except CKE are ”Don’t Care” during SELF REFRESH.

The procedure for exiting self refresh CK must be stable prior to CKE going back HIGH. DDR SDRAM must have NOP commands issued for tXSNR because time is required for the completion of any internal refresh in progress. A simple algorithm for meeting both refresh and DLL requirements is to apply NOPs for 200 clock cycles before applying any other command.

The use of SELF REFRESH mode introduces the possibility that an internally timed event can be missed when CKE is raised for exit from self refresh mode. Upon exit from SELF REFRESH an extra auto refresh command is recommended.

OPERATIONS BANK/ROW ACTIVATION

READ BURST-REQUIRED CAS LATENCIES

WRITE TO WRITE -- Max tDQSS, NON CONSECUTIVE

RANDOM WRITE CYCLES -- Max tDQSS

WRITE TO READ -- Max tDQSS, NON--INTERRUPTING

WRITE TO READ -- Max tDQSS, INTERRUPTING

WRITE TO READ -- Max tDQSS, ODD NUMBER OF DATA,INTERRUPTING

WRITE TO PRECHARGE -- Max tDQSS, NON--INTERRUPTING

WRITE TO PRECHARGE -- Max tDQSS, INTERRUPTING

WRITE TO PRECHARGE -- Max tDQSS,ODD NUMBER OF DATA, INTERRUPTING

Double data rate (ddr)

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