W3E32M64S-XBX
White Electronic Designs
location within the block. The programmed burst length
applies to both READ and WRITE bursts.
REGISTER DEFINITION
MODE REGISTER
BURST TYPE
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).
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
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.
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.
Mode register bitsA0-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.
Reserved states should not be used as unknown operation
or incompatibility with future versions may result.
BURST LENGTH
OPERATING MODE
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.
The normal operating mode is selected by issuing a MODE
REGISTER SET command with bits A7-A12 each set to
zero, and bitsA0-A6 set to the desired values.ADLL reset
is initiated by issuing a MODE REGISTER SET command
with bitsA7 andA9-A12 each set to zero, bitA8 set to one,
and bits A0-A6 set to the desired values. Although not
required, JEDEC specifications recommend when a LOAD
MODE REGISTER command is issued to reset the DLL, it
should always be followed by a LOAD MODE REGISTER
command to select normal operating mode.
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; byA2-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
All other combinations of values for A7-A12 are reserved
for future use and/or test modes. Test modes and reserved
states should not be used because unknown operation or
incompatibility with future versions may result.
EXTENDED MODE REGISTER
The extended mode register controls functions beyond
those controlled by the mode register; these additional
functions are DLL enable/disable, output drive strength,
February 2007
Rev. 4
5
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