iPEM
4.2 Gb SDRAM-DDR2
Austin Semiconductor, Inc.
AS4DDR264M64PBG1
READ COMMAND
The READ command is used to initiate a burst read access
to an active row. The value on the BA2–BA0 inputs selects
the bank, and the address provided on inputs A0–i (where
i = A9) selects the starting column location. The value on
input A10 determines whether or not auto precharge is used.
If auto precharge is selected, the row being accessed will
be precharged at the end of the READ burst; if auto precharge
is not selected, the row will remain open for subsequent
accesses.
FIGURE 11 - READ COMMAND
READ OPERATION
READ bursts are initiated with a READ command. The starting
column and bank addresses are provided with the READ
command and auto precharge is either enabled or disabled
for that burst access. If auto precharge is enabled, the row
being accessed is automatically precharged at the
completion of the burst. If auto precharge is disabled, the
row will be left open after the completion of the burst.
CK#
CK
CKE
CS#
During READ bursts, the valid data-out element from the
starting column address will be available READ latency (RL)
clocks later. RL is defined as the sum of AL and CL; RL = AL
+ CL. The value for AL and CL are programmable via the MR
and EMR commands, respectively. Each subsequent data-
out element will be valid nominally at the next positive or
negative clock edge (i.e., at the next crossing of CK and
CK#).
RAS#
CAS#
WE#
ADDRESS
Col
DQS/DQS# is driven by the DDR2 SDRAM along with output
data. The initial LOW state on DQS and HIGH state on DQS#
is known as the read preamble (tRPRE). The LOW state on
DQS and HIGH state on DQS# coincident with the last data-
out element is known as the read postamble (tRPST).
ENABLE
A10
AUTO PRECHARGE
BANK ADDRESS
DISABLE
Bank
Upon completion of a burst, assuming no other commands
have been initiated, the DQ will go High-Z.
DON’T CARE
Data from any READ burst may be concatenated with data
from a subsequent READ command to provide a continuous
flow of data. The first data element from the new burst follows
the last element of a completed burst. The new READ
command should be issued x cycles after the first READ
command, where x equals BL / 2 cycles.
Austin Semiconductor, Inc.
● Austin, Texas ● 512.339.1188 ● www.austinsemiconductor.com
AS4DDR264M64PBG1
Rev. 0.5 06/08
16
AUSTIN [ AUSTIN SEMICONDUCTOR ]
