SDRAM
AS4SD4M16
Austin Semiconductor, Inc.
CAS Latency
Reserved states should not be used as unknown op-
The CAS latency is the delay, in clock cycles, between eration or incompatibility with future versions may result.
the registration of a READ command and the availability of the Operating Mode
first piece of output data. The latency can be set to two or three
clocks.
The normal operating mode is selected by setting
M7and M8 to zero; the other combinations of values for M7
If a READ command is registered at clock edge n, and and M8 are reserved for future use and/or test modes. The
the latency is m clocks, the data will be available by clock edge programmed burst length applies to both READ and WRITE
n+m. The DQs will start driving as a result of the clock edge one bursts.
cycle earlier (n + m - 1), and provided that the relevant access
Test modes and reserved states should not be used
times are met, the data will be valid by clock edge n + m. For because unknown operation or incompatibility with future ver-
example, assuming that the clock cycle time is such that all sions may result.
relevant access times are met, if a READ command is registered Write Burst Mode
at T0 and the latency is programmed to two clocks, the DQs will
When M9 = 0, the burst length programmed via M0-
start driving after T1 and the data will be valid by T2, as shown M2 applies to both READ and WRITE bursts; when M9 = 1,
in Figure 2. Table 2 below indicates the operating frequencies at the programmed burst length applies to READ bursts, but write
which each CAS latency setting can be used.
accesses are single-location (nonburst) accesses.
Table 2
CAS LATENCY
T0
T1
T2
T3
CLK
ALLOWABLE OPERATING FREQUENCY
(MHz)
READ
NOP
NOP
COMMAMD
DQ
CAS LATENCY = 2
CAS LATENCY = 3
≤ 125
SPEED
-8
-10
tLZ
tOH
DOUT
≤ 83
≤ 66
≤ 100
tAC
CAS Latency = 2
T0
T1
T2
T3
T4
CLK
NOP
READ
NOP
tLZ
NOP
tOH
COMMAMD
DQ
DOUT
tAC
CAS Latency = 3
UNDEFINED
DON’T CARE
Figure 2
CAS LATENCY
Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.
AS4SD4M16
Rev. 1.5 10/01
8
AUSTIN [ AUSTIN SEMICONDUCTOR ]
