64Mb: x4, x8, x16 SDRAM
READ Operation
The DQM signal must be de-asserted prior to the WRITE command (DQM latency is
zero clocks for input buffers) to ensure that the written data is not masked. Figure 20
(page 50) shows where, due to the clock cycle frequency, bus contention is avoided
without having to add a NOP cycle, while Figure 21 (page 51) shows the case where an
additional NOP cycle is required.
A fixed-length READ burst may be followed by or truncated with a PRECHARGE com-
mand to the same bank, provided that auto precharge was not activated. The PRE-
CHARGE command should be issued x cycles before the clock edge at which the last de-
sired data element is valid, where x = CL - 1. This is shown in Figure 22 (page 51) for
each possible CL; data element n + 3 is either the last of a burst of four or the last de-
sired data element of a longer burst. Following the PRECHARGE command, a subse-
quent command to the same bank cannot be issued until tRP is met. Note that part of
the row precharge time is hidden during the access of the last data element(s).
In the case of a fixed-length burst being executed to completion, a PRECHARGE com-
mand issued at the optimum time (as described above) provides the same operation
that would result from the same fixed-length burst with auto precharge. The disadvant-
age of the PRECHARGE command is that it requires that the command and address
buses be available at the appropriate time to issue the command. The advantage of the
PRECHARGE command is that it can be used to truncate fixed-length or continuous
page bursts.
Figure 20: READ-to-WRITE
T0
T1
T2
T3
T4
CLK
DQM
READ
NOP
NOP
NOP
Command
Address
WRITE
Bank,
Col n
Bank,
Col b
t
CK
t
HZ
DOUT
DIN
DQ
t
DS
Transitioning data
Don’t Care
1. CL = 3. The READ command can be issued to any bank, and the WRITE command can be
to any bank. If a burst of one is used, DQM is not required.
Note:
PDF: 09005aef80725c0b
64mb_x4x8x16_sdram.pdf - Rev. U 05/13 EN
Micron Technology, Inc. reserves the right to change products or specifications without notice.
50
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