White Electronic Designs
February 2005
Rev. 3
WEDPN16M72V-XB2X
4
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performed. After the AUTO REFRESH cycles are complete,
the SDRAM is ready for Mode Register programming.
Because the Mode Register will power up in an unknown
state, it should be loaded prior to applying any operational
command.
REGISTER DEFINITION
MODE REGISTER
The Mode Register is used to defi ne the specifi c mode
of operation of the SDRAM. This defi nition includes the
selec-tion of a burst length, a burst type, a CAS latency,
an operating mode and a write burst mode, as shown in
Figure 3. The Mode Register is programmed via the LOAD
MODE REGISTER command and will retain the stored
information until it is programmed again or the device
loses power.
Mode register bits M0-M2 specify the burst length, M3
specifi es the type of burst (sequential or interleaved),
M4-M6 specify the CAS latency, M7 and M8 specify the
operating mode, M9 specifi es the WRITE burst mode,
and M10 and M11 are reserved for future use. Address
A12 (M12) is undefi ned but should be driven LOW during
loading of the mode register.
The Mode Register must be loaded when all banks are
idle, and the controller must wait the specifi ed time before
initiating the subsequent operation. Violating either of these
requirements will result in unspecifi ed operation.
BURST LENGTH
Read and write accesses to the 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 1, 2, 4
or 8 locations are available for both the sequential and the
interleaved burst types, and a full-page burst is available
for the sequential type. The full-page burst is used in
conjunction with the BURST TERMINATE command to
generate arbitrary burst lengths.
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
A1-8 when the burst length is set to two; by A2-8 when
The 1Gb SDRAM is designed to operate in 3.3V, low-power
memory systems. An auto refresh mode is provided, along
with a power-saving, power-down mode.
All inputs and outputs are LVTTL compatible. SDRAMs offer
substantial advances in DRAM operating performance,
including the ability to synchronously burst data at a high
data rate with automatic column-address generation,
the ability to interleave between internal banks in order
to hide precharge time and the capability to randomly
change column addresses on each clock cycle during a
burst access.
FUNCTIONAL DESCRIPTION
Read and write accesses to the 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 registration of an
ACTIVE command which is then followed by a READ or
WRITE command. The address bits registered coincident
with the ACTIVE command are used to select the bank
and row to be accessed (BA0 and BA1 select the bank,
A0-12 select the row). The address bits (A0-8) registered
coincident with the READ or WRITE command are used to
select the starting column location for the burst access.
Prior to normal operation, the SDRAM must be initialized.
The following sections provide detailed information
covering device initialization, register defi nition, command
descriptions and device operation.
INITIALIZATION
SDRAMs must be powered up and initialized in a predefi ned
manner. Operational procedures other than those specifi ed
may result in undefi ned operation. Once power is applied
to V
CC
and V
CCQ
(simultaneously) and the clock is stable
(stable clock is defi ned as a signal cycling within timing
constraints specified for the clock pin), the SDRAM
requires a 100µs delay prior to issuing any command
other than a COMMAND INHIBIT or a NOP. Starting at
some point during this 100µs period and continuing at
least through the end of this period, COMMAND INHIBIT
or NOP commands should be applied.
Once the 100µs delay has been satisfi ed with at least
one COMMAND INHIBIT or NOP command having been
applied, a PRECHARGE command should be applied. All
banks must be precharged, thereby placing the device in
the all banks idle state.
Once in the idle state, two AUTO REFRESH cycles must be
