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White Electronic Designs Corporation (602) 437-1520 www.whiteedc.com
White Electronic Designs
WEDPN4M72V-XB2X
January 2005
Rev. 2
White Electronic Designs Corp. reserves the right to change products or specications without notice.
FIGURE 4 – CAS LATENCY
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 I/Os 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 I/Os will start driving
after T1 and the data will be valid by T2. Table 2 below
indicates the operating frequencies at which each CAS
latency setting can be used.
Reserved states should not be used as unknown operation
or incompatibility with future versions may result.
OPERATING MODE
The normal operating mode is selected by setting M7and
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.
A1-7 when the burst length is set to two; by A2-7 when
the burst length is set to four; and by A3-7 when the burst
length is set to eight. The remaining (least signicant)
address bit(s) is (are) used to select the starting location
within the block. Full-page bursts wrap within the page if
the boundary is reached.
BURST TYPE
Accesses within a given burst may be programmed to be
either sequential or interleaved; this is referred to as the
burst type and is selected via bit M3.
The ordering of accesses within a burst is determined by
the burst length, the burst type and the starting column
address, as shown in Table 1.
CAS LATENCY
The CAS latency is the delay, in clock cycles, between
the registration of a READ command and the availability
of the rst piece of output data. The latency can be set to
two or three clocks.
CK
I/O
T2
T1
T3
T0
CAS Latency = 3
LZ
DOUT
tOH
t
COMMAND
NOP
READ
tAC
NOP
T4
NOP
DON’T CARE
UNDEFINED
CK
I/O
T2
T1
T3
T0
CAS Latency = 2
LZ
DOUT
OH
t
COMMAND
READ
tAC
NOP