128Mb: x4, x8, x16
SDRAM
CAS Latency
Operating Mode
The CAS latency is the delay, in clock cycles, between
the registration of a READ command and the availability
of the first piece of output data. The latency can be set to
two or three clocks.
The normal operating mode is selected by setting M7
and M8 to zero; the other combinations of values for M7
and M8 are reserved for future use and/or test modes.
The programmed burst length applies to both READ and
WRITE bursts.
Test modes and reserved states should not be used
because unknown operation or incompatibility with fu-
ture versions may result.
If a READ command is registered at clock edge n, and
the latency is m clocks, the data will be available by clock
edge n + m. The DQs will start driving as a result of the
clock edge one cycle earlier (n + m - 1), and provided that
the relevant access times are met, the data will be valid by
clock edge n + m. For example, assuming that the clock
cycle time is such that all relevant access times are met,
if a READ command is registered at T0 and the latency is
programmed to two clocks, the DQs will start driving
after T1 and the data will be valid by T2, as shown in
Figure 2. Table 2 below indicates the operating frequen-
cies at which each CAS latency setting can be used.
Reserved states should not be used as unknown op-
eration or incompatibility with future versions
may result.
Write Burst Mode
When M9 = 0, the burst length programmed via
M0-M2 applies to both READ and WRITE bursts; when
M9 = 1, the programmed burst length applies to
READ bursts, but write accesses are single-location
(nonburst) accesses.
Table 2
CAS Latency
ALLOWABLE OPERATING
FREQUENCY (MHz)
T0
T1
T2
T3
CLK
CAS
LATENCY = 2
CAS
LATENCY = 3
SPEED
COMMAND
READ
NOP
t
NOP
t
-7E
-75
-8E
≤ 133
≤ 100
≤ 100
≤ 143
≤ 133
≤ 125
LZ
OH
D
OUT
DQ
t
AC
CAS Latency = 2
T0
T1
T2
T3
T4
CLK
COMMAND
READ
NOP
NOP
NOP
t
t
LZ
OH
D
OUT
DQ
t
AC
CAS Latency = 3
DON’T CARE
UNDEFINED
Figure 2
CAS Latency
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.
11
MICRON [ MICRON TECHNOLOGY ]
