W3E16M72SR-XBX Datasheet, PDF (5/16 Pages) White Electronic Designs Corporation – 16Mx72 Registered DDR SDRAM

White Electronic Designs
W3E16M72SR-XBX
Once the 200µs delay has been satisfied, a DESELECT
or NOP command should be applied, and CKE should
be brought HIGH. Following the NOP command, a
PRECHARGE ALL command should be applied. Next a
LOAD MODE REGISTER command should be issued for
the extended mode register (BA1 LOW and BA0 HIGH)
to enable the DLL, followed by another LOAD MODE
REGISTER command to the mode register (BA0/BA1
both LOW) to reset the DLL and to program the operating
parameters. Two-hundred clock cycles are required
between the DLL reset and any READ command. A
PRECHARGE ALL command should then be applied,
placing the device in the all banks idle state.
Once in the idle state, two AUTO REFRESH cycles must
be performed (tRFC must be satisfied.) Additionally, a LOAD
MODE REGISTER command for the mode register with
the reset DLL bit deactivated (i.e., to program operating
parameters without resetting the DLL) is required.
Following these requirements, the DDR SDRAM is ready
for normal operation.
REGISTER DEFINITION
MODE REGISTER
The Mode Register is used to define the specific mode of
operation of the DDR SDRAM. This definition includes the
selection of a burst length, a burst type, a CAS latency,
and an operating mode, as shown in Figure 3. The Mode
Register is programmed via the MODE REGISTER SET
command (with BA0 = 0 and BA1 = 0) and will retain
the stored information until it is programmed again or
the device loses power. (Except for bit A8 which is self
clearing).
Reprogramming the mode register will not alter the contents
of the memory, provided it is performed correctly. The
Mode Register must be loaded (reloaded) when all banks
are idle and no bursts are in progress, and the controller
must wait the specified time before initiating the subsequent
operation. Violating either of these requirements will result
in unspecified operation.
Mode register bits A0-A2 specify the burst length, A3
specifies the type of burst (sequential or interleaved),
A4-A6 specify the CAS latency, and A7-A12 specify the
operating mode.
BURST LENGTH
Read and write accesses to the DDR SDRAM are burst
oriented, with the burst length being programmable,
as shown in Figure 3. The burst length determines
the maximum number of column locations that can be
accessed for a given READ or WRITE command. Burst
lengths of 2, 4 or 8 locations are available for both the
sequential and the interleaved burst types.
Reserved states should not be used, as unknown operation
or incompatibility with future versions may result.
When a READ or WRITE command is issued, a block of
columns equal to the burst length is effectively selected. All
accesses for that burst take place within this block, meaning
that the burst will wrap within the block if a boundary is
reached. The block is uniquely selected by A1-Ai when the
burst length is set to two; by A2-Ai when the burst length
is set to four (where Ai is the most significant column
address for a given configuration); and by A3-Ai when the
burst length is set to eight. The remaining (least significant)
address bit(s) is (are) used to select the starting location
within the block. The programmed burst length applies to
both READ and WRITE bursts.
BURST TYPE
Accesses within a given burst may be programmed to be
either sequential or interleaved; this is referred to as the
burst type and is selected via bit M3.
The ordering of accesses within a burst is determined by
the burst length, the burst type and the starting column
address, as shown in Table 1.
READ LATENCY
The READ latency is the delay, in clock cycles, between
the registration of a READ command and the availability
of the first bit of output data. The latency can be set to 2
or 2.5 clocks.
If a READ command is registered at clock edge n, and the
latency is m clocks, the data will be available by clock edge
n+m. Table 2 below indicates the operating frequencies at
which each CAS latency setting can be used.
Reserved states should not be used as unknown operation
or incompatibility with future versions may result.
February 2005
Rev. 2
5
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