IBM0625164GT3B IBM0625404GT3B
IBM06254B4GT3B IBM0625804GT3B
Advance
256Mb Double Data Rate Synchronous DRAM
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. A precharge of the bank/row that is addressed with the
Read or Write command is automatically performed upon completion of the Read or Write burst. Auto Pre-
charge is nonpersistent in that it is either enabled or disabled for each individual Read or Write command.
Auto Precharge ensures that the precharge is initiated at the earliest valid stage within a burst. The user must
not issue another command to the same bank until the precharge (t ) is completed. This is determined as if
RP
an explicit Precharge command was issued at the earliest possible time, as described for each burst type in
the Operation section of this data sheet.
Burst Terminate
The Burst Terminate command is used to truncate read bursts (with Auto Precharge disabled). The most re-
cently registered Read command prior to the Burst Terminate command is truncated, as shown in the Opera-
tion section of this data sheet. Write burst cycles are not to be terminated with the Burst Terminate command.
Auto Refresh
Auto Refresh is used during normal operation of the DDR SDRAM and is analogous to CAS Before RAS
(CBR) Refresh in previous DRAM types. This command is nonpersistent, so it must be issued each time a
refresh is required.
The refresh addressing is generated by the internal refresh controller. This makes the address bits “Don’t
Care” during an Auto Refresh command. The 256Mb DDR SDRAM requires Auto Refresh cycles at an aver-
age periodic interval of 7.8µs (maximum).
To allow for improved efficiency in scheduling and switching between tasks, some flexibility in the absolute
refresh interval is provided. A maximum of eight Auto Refresh commands can be posted in the system,
meaning that the maximum absolute interval between any Auto Refresh command and the next Auto Refresh
command is 9 * 7.8µs (70.2µs). This maximum absolute interval is short enough to allow for DLL updates
internal to the DDR SDRAM to be restricted to Auto Refresh cycles, without allowing too much drift in t
between updates.
AC
Self Refresh
The Self Refresh command can be used to retain data in the DDR SDRAM, even if the rest of the system is
powered down. When in the self refresh mode, the DDR SDRAM retains data without external clocking. The
Self Refresh command is initiated as an Auto Refresh command coincident with CKE transitioning low. The
DLL is automatically disabled upon entering Self Refresh, and is automatically enabled upon exiting Self
Refresh (200 clock cycles must then occur before a Read command can be issued). Input signals except
CKE (low) are “Don’t Care” during Self Refresh operation.
The procedure for exiting self refresh requires a sequence of commands. CK (and CK) must be stable prior to
CKE returning high. Once CKE is high, the SDRAM must have NOP commands issued for t
because
XSNR
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.
©IBM Corporation. All rights reserved.
Use is further subject to the provisions at the end of this document.
29L0011.E36997
10/99
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