64Mb: x32
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 turn-
around limitations). The WRITE burst may be initiated
on the clock edge immediately following the last (or last
desired) data element from the READ burst, provided
that I/O contention can be avoided. In a given system
design, there may be a possibility that the device driv-
ing 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 buff-
ers) to suppress data-out from the READ. Once the
WRITE command is registered, the DQs will go High-Z
(or remain 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
command. If not, the second WRITE will be an invalid
WRITE. For example, if DQM was low during T4 in Fig-
ure 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 al-
lows for bus contention to be avoided without adding a
NOP cycle, and Figure 10 shows the case where the
additional NOP is needed.
Figure 9
READ to WRITE
T0
T1
T2
T3
T4
CLK
DQM
READ
NOP
NOP
NOP
WRITE
COMMAND
ADDRESS
Figure 10
READ to WRITE with
Extra Clock Cycle
BANK,
COL n
BANK,
COL b
t
CK
t
HZ
T0
T1
T2
T3
T4
T5
DOUT n
DIN b
CLK
DQ
t
DS
DON’T CARE
DQM
READ
NOP
NOP
NOP
NOP
WRITE
COMMAND
ADDRESS
NOTE:
A CAS latency of three is used for illustration. The READ
command may be to any bank, and the WRITE command
BANK,
BANK,
COL n
COL b
t
HZ
DOUT
n
DIN
b
DQ
t
DS
DON’T CARE
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.
64Mb: x32 SDRAM
64MSDRAMx32_5.p65 – Rev. B; Pub. 6/02
MicronTechnology,Inc.,reservestherighttochangeproductsorspecificationswithoutnotice.
©2002,MicronTechnology,Inc.
16
MICRON [ MICRON TECHNOLOGY ]
