iPEM
4.2 Gb SDRAM-DDR2
Austin Semiconductor, Inc.
AS4DDR264M64PBG1
DESELECT
The DESELECT function (CS# HIGH) prevents new
commands from being executed by the DDR2 SDRAM. The
DDR2 SDRAM is effectively deselected. Operations already
in progress are not affected.
A subsequent ACTIVE command to a different row in the
same bank can only be issued after the previous active row
has been closed (precharged). The minimum time interval
between successive ACTIVE commands to the same bank
is defined by tRC
NO OPERATION (NOP)
A subsequent ACTIVE command to another bank can be
issued while the first bank is being accessed, which results
in a reduction of total row-access overhead. The minimum
time interval between successive ACTIVE commands to
different banks is defined by tRRD.
The NO OPERATION (NOP) command is used to instruct
the selected DDR2 SDRAM to perform a NOP (CS# is LOW;
RAS#, CAS#, and WE are HIGH). This prevents unwanted
commands from being registered during idle or wait states.
Operations already in progress are not affected.
LOAD MODE (LM)
The mode registers are loaded via inputs BA2–BA0, and
A12–A0. BA2–BA0 determine which mode register will be
programmed. See “Mode Register (MR)”. The LM command
can only be issued when all banks are idle, and a
subsequent execute able command cannot be issued until
tMRD is met.
FIGURE 10 - ACTIVE COMMAND
CK#
CK
BANK/ROW ACTIVATION
ACTIVE COMMAND
CKE
CS#
The ACTIVE command is used to open (or activate) a row in
a particular bank for a subsequent access. The value on the
BA2–BA0 inputs selects the bank, and the address provided
on inputs A12–A0 selects the row. This row remains active
(or open) for accesses until a PRECHARGE command is
issued to that bank. A PRECHARGE command must be
issued before opening a different row in the same bank.
RAS#
CAS#
WE#
ACTIVE OPERATION
Before any READ or WRITE commands can be issued to a
bank within the DDR2 SDRAM, a row in that bank must be
opened (activated), even when additive latency is used. This
is accomplished via the ACTIVE command, which selects
both the bank and the row to be activated.
Row
ADDRESS
BANK ADDRESS
Bank
After a row is opened with an ACTIVE command, a READ or
WRITE command may be issued to that row, subject to the
tRCD specification. tRCD (MIN) should be divided by the
clock period and rounded up to the next whole number to
determine the earliest clock edge after the ACTIVE command
on which a READ or WRITE command can be entered. The
same procedure is used to convert other specification limits
from time units to clock cycles. For example, a tRCD (MIN)
specification of 20ns with a 266 MHz clock (tCK = 3.75ns)
results in 5.3 clocks, rounded up to 6.
DON’T CARE
Austin Semiconductor, Inc.
● Austin, Texas ● 512.339.1188 ● www.austinsemiconductor.com
AS4DDR264M64PBG1
Rev. 0.5 06/08
15
AUSTIN [ AUSTIN SEMICONDUCTOR ]
