IBMN625404GT3B
IBMN625804GT3B
256Mb Double Data Rate Synchronous DRAM
Preliminary
Functional Description
The 256Mb DDR SDRAM is a high-speed CMOS, dynamic random-access memory containing 268, 435, 456
bits. The 256Mb DDR SDRAM is internally configured as a quad-bank DRAM.
The 256Mb DDR SDRAM uses a double-data-rate architecture to achieve high-speed operation. The double-
data-rate architecture is essentially a 2n prefetch architecture, with an interface designed to transfer two data
words per clock cycle at the I/O pins. A single read or write access for the 256Mb DDR SDRAM consists of a
single 2n-bit wide, one clock cycle data transfer at the internal DRAM core and two corresponding n-bit wide,
one-half clock cycle data transfers at the I/O pins.
Read and write accesses to the DDR SDRAM are burst oriented; accesses start at a selected location and
continue for a programmed number of locations in a programmed sequence. Accesses begin with the regis-
tration of an Active command, which is then followed by a Read or Write command. The address bits regis-
tered coincident with the Active command are used to select the bank and row to be accessed (BA0, BA1
select the bank; A0-A12 select the row). The address bits registered coincident with the Read or Write com-
mand are used to select the starting column location for the burst access.
Prior to normal operation, the DDR SDRAM must be initialized. The following sections provide detailed infor-
mation covering device initialization, register definition, command descriptions and device operation.
Initialization
Only one of the following two conditions must be met.
• No power sequencing is specified during power up or power down given the following criteria:
V
V
and V
meets the specification
are driven from a single power converter output
DDQ
DD
TT
A minimum resistance of 42 ohms limits the input current from the VTT supply into any pin and
tracks V /2
V
REF
DDQ
or
• The following relationships must be followed:
V
V
V
is driven after or with V such that V
< V + 0.3V
DDQ DD
DDQ
DD
is driven after or with V
such that V < V
+ 0.3V
TT
DDQ
TT
DDQ
is driven after or with V
such that V
< V
+ 0.3V
DDQ
REF
DDQ
REF
The DQ and DQS outputs are in the High-Z state, where they remain until driven in normal operation (by a
read access). After all power supply and reference voltages are stable, and the clock is stable, the DDR
SDRAM requires a 200µs delay prior to applying an executable command.
Once the 200µs delay has been satisfied, a Deselect or NOP command should be applied, and CKE must be
brought HIGH. Following the NOP command, a Precharge ALL command must be applied. Next a Mode
Register Set command must be issued for the Extended Mode Register, to enable the DLL, then a Mode
Register Set command must be issued for the Mode Register, to reset the DLL, and to program the operating
parameters. 200 clock cycles are required between the DLL reset and any read command. A Precharge ALL
command should be applied, placing the device in the “all banks idle” state.
Once in the idle state, two auto refresh cycles must be performed. Additionally, a Mode Register Set com-
mand 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 opera-
tion.
©IBM Corporation. All rights reserved.
Use is further subject to the provisions at the end of this document.
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