128Mb: x4, x8, x16
SDRAM
Data from any READ burst may be truncated with a
subsequent WRITE command, and data from a fixed-
length READ burst may be immediately followed by data
from a WRITE command (subject to bus turnaround
limitations). The WRITE burst may be initiated on the
clock edge immediately following the last (or last de-
sired)dataelementfromtheREADburst, providedthatI/
O contention can be avoided. In a given system design,
there may be a possibility that the device driving the
input data will go Low-Z before the SDRAM DQs go High-
Z. In this case, at least a single-cycle delay should occur
between the last read data and the WRITE command.
The DQM input is used to avoid I/O contention, as
shown in Figures 9 and 10. The DQM signal must be
asserted (HIGH) at least two clocks prior to the WRITE
command (DQM latency is two clocks for output buffers)
to suppress data-out from the READ. Once the WRITE
command is registered, the DQs will go High-Z (or re-
main High-Z), regardless of the state of the DQM signal,
provided the DQM was active on the clock just prior to
the WRITE command that truncated the READ com-
mand. If not, the second WRITE will be an invalid WRITE.
For example, if DQM was LOW during T4 in Figure 10,
then the WRITEs at T5 and T7 would be valid, while the
WRITE at T6 would be invalid.
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 9 shows the case where the clock frequency allows
for bus contention to be avoided without adding a NOP
cycle, and Figure 10 shows the case where the additional
NOP is needed.
T0
T1
T2
T3
T4
T5
T0
T1
T2
T3
T4
CLK
CLK
DQM
DQM
READ
NOP
NOP
NOP
NOP
WRITE
COMMAND
ADDRESS
READ
NOP
NOP
NOP
WRITE
COMMAND
ADDRESS
BANK,
COL b
BANK,
COL n
BANK,
COL n
BANK,
COL b
t
HZ
t
CK
D
OUT
n
DIN
b
DQ
t
HZ
t
DS
D
OUT
n
DIN b
DQ
t
DS
NOTE:
A CAS latency of three is used for illustration. The READ command
may be to any bank, and the WRITE command may be to any bank.
NOTE:
A CAS latency of three is used for illustration. The READ
command may be to any bank, and the WRITE command
may be to any bank. If a burst of one is used, then DQM is
not required.
DON’T CARE
Figure 10
READ to WRITE With
Extra Clock Cycle
Figure 9
READ to WRITE
128Mb: x4, x8, x16 SDRAM
128MSDRAM_E.p65 – Rev. E; Pub. 1/02
Micron Technology, Inc., reserves the right to change products or specifications without notice.
©2001, Micron Technology, Inc.
19
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