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CAS Latency (CL)
the delay, in clock cycles, between the registration of a READ command and the availa-
bility of the first bit of output data. The CL can be set to 3, 4, 5, 6, or 7 clocks, depending
on the speed grade option being used.
DDR2 SDRAM does not support any half-clock latencies. Reserved states should not be
used as an unknown operation otherwise incompatibility with future versions may result.
DDR2 SDRAM also supports a feature called posted CAS additive latency (AL). This fea-
ture allows the READ command to be issued prior to tRCD (MIN) by delaying the
internal command to the DDR2 SDRAM by AL clocks. The AL feature is described in
a READ command is registered at clock edge n, and the CL is m clocks, the data will be
available nominally coincident with clock edge n + m (this assumes AL = 0).
Figure 35: CL
DO
n + 3
DO
n + 2
DO
n + 1
CK
CK#
Command
DQ
DQS, DQS#
CL = 3 (AL = 0)
READ
T0
T1
T2
Don’t care
Transitioning data
NOP
DO
n
T3
T4
T5
NOP
T6
NOP
DO
n + 3
DO
n + 2
DO
n + 1
CK
CK#
Command
DQ
DQS, DQS#
CL = 4 (AL = 0)
READ
T0
T1
T2
NOP
DO
n
T3
T4
T5
NOP
T6
NOP
Notes: 1. BL = 4.
2. Posted CAS# additive latency (AL) = 0.
3. Shown with nominal tAC, tDQSCK, and tDQSQ.
1Gb: x4, x8, x16 DDR2 SDRAM
Mode Register (MR)
PDF: 09005aef821ae8bf
Rev. P 1/09 EN
81
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2004 Micron Technology, Inc. All rights reserved.