1Gb: x8, x16 Automotive DDR2 SDRAM - Micron

READ
1Gb: x8, x16 Automotive DDR2 SDRAM
READ
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.
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 (at the next crossing of CK and CK#). Figure 43
(page 89) shows examples of RL based on different AL and CL settings.
DQS/DQS# is driven by the DDR2 SDRAM along with output data. The initial LOW state
on DQS and the HIGH state on DQS# are known as the read preamble ( tRPRE). The
LOW state on DQS and the HIGH state on DQS# coincident with the last data-out element
are known as the read postamble ( tRPST). Upon completion of a burst, assuming no other commands have been initiated, the DQ
will go High-Z. A detailed explanation of tDQSQ (valid data-out skew), tQH (data-out
window hold), and the valid data window are depicted in Figure 52 (page 97) and Figure
53 (page 98). A detailed explanation of tDQSCK (DQS transition skew to CK) and
tAC (data-out transition skew to CK) is shown in Figure 54 (page 99).
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 (see Figure 44
(page 90)).
Nonconsecutive read data is illustrated in Figure 45 (page 91). Full-speed random read
accesses within a page (or pages) can be performed. DDR2 SDRAM supports the use of
concurrent auto precharge timing (see Table 41 (page 94)).
DDR2 SDRAM does not allow interrupting or truncating of any READ burst using BL = 4
operations. Once the BL = 4 READ command is registered, it must be allowed to complete
the entire READ burst. However, a READ (with auto precharge disabled) using BL =
8 operation may be interrupted and truncated only by another READ burst as long as
the interruption occurs on a 4-bit boundary due to the 4n prefetch architecture of
DDR2 SDRAM. As shown in Figure 46 (page 92), READ burst BL = 8 operations may
not be interrupted or truncated with any other command except another READ command.
Data from any READ burst must be completed before a subsequent WRITE burst is allowed.
An example of a READ burst followed by a WRITE burst is shown in Figure 47
(page 92). The tDQSS (NOM) case is shown ( tDQSS [MIN] and tDQSS [MAX] are defined
in Figure 55 (page 101)).
PDF: 09005aef840eff89
1gbddr2_ait_aat.pdf – Rev. C 7/11 EN 88 Micron Technology, Inc. reserves the right to change products or specifications without notice.
� 2010 Micron Technology, Inc. All rights reserved.