W332M72V-XSBX
White Electronic Designs
two clocks earlier. If a given DQM signal was registered
HIGH, the corresponding I/Os will be High-Z two clocks
later; if the DQM signal was registered LOW, the I/Os will
provide valid data.
issue another command to the same bank until the precharge
time (tRP) is completed. This is determined as if an explicit
PRECHARGE command was issued at the earliest possible
time.
WRITE
BURST TERMINATE
The WRITE command is used to initiate a burst write
access to an active row. The value on the BA0, BA1 inputs
selects the bank, and the address provided on inputsA0-9
selects the starting column location. The value on inputA10
determines whether or notAUTO PRECHARGE is used. If
AUTO PRECHARGE is selected, the row being accessed
will be precharged at the end of the WRITE burst; ifAUTO
PRECHARGE is not selected, the row will remain open for
subsequent accesses. Input data appearing on the I/Os
is written to the memory array subject to the DQM input
logic level appearing coincident with the data. If a given
DQM signal is registered LOW, the corresponding data
will be written to memory; if the DQM signal is registered
HIGH, the corresponding data inputs will be ignored, and a
WRITE will not be executed to that byte/column location.
The BURST TERMINATE command is used to truncate
either fixed-length or full-page bursts. The most recently
registered READ or WRITE command prior to the BURST
TERMINATE command will be truncated.
AUTO REFRESH
AUTO REFRESH is used during normal operation of
the SDRAM and is analagous to CAS#-BEFORE-RAS#
(CBR) REFRESH in conventional DRAMs. This command
is nonpersistent, so it must be issued each time a refresh
is required.
The addressing is generated by the internal refresh
controller. This makes the address bits “Don’t Care”
during an AUTO REFRESH command. Each 512Mb
SDRAM requires 8,192 AUTO REFRESH cycles every
refresh period (tREF). Providing a distributed AUTO
REFRESH command will meet the refresh requirement
and ensure that each row is refreshed.Alternatively, 8,192
AUTO REFRESH commands can be issued in a burst at
the minimum cycle rate (tRC), once every refresh period
(tREF).
PRECHARGE
The PRECHARGE command is used to deactivate the
open row in a particular bank or the open row in all banks.
The bank(s) will be available for a subsequent row access
a specified time (tRP) after the PRECHARGE command
is issued. Input A10 determines whether one or all banks
are to be precharged, and in the case where only one
bank is to be precharged, inputs BA0, BA1 select the bank.
Otherwise BA0, BA1 are treated as “Don’t Care.” Once a
bank has been precharged, it is in the idle state and must
be activated prior to any READ or WRITE commands being
issued to that bank.
SELF REFRESH*
The SELF REFRESH command can be used to retain data
in the SDRAM, even if the rest of the system is powered
down. When in the self refresh mode, the SDRAM retains
data without external clocking. The SELF REFRESH
command is initiated like an AUTO REFRESH command
except CKE is disabled (LOW). Once the SELF REFRESH
command is registered, all the inputs to the SDRAM
become “Don’t Care,” with the exception of CKE, which
must remain LOW.
AUTO PRECHARGE
AUTO PRECHARGE is a feature which performs the same
individual-bank PRECHARGE function described above,
without requiring an explicit command. This is accomplished
by using A10 to enable AUTO PRECHARGE in conjunction
with a specific READ or WRITE command. A precharge of
the bank/row that is addressed with the READ or WRITE
command is automatically performed upon completion of
the READ or WRITE burst, except in the full-page burst
mode, where AUTO PRECHARGE does not apply. AUTO
PRECHARGE is nonpersistent in that it is either enabled or
disabled for each individual READ or WRITE command.
Once self refresh mode is engaged, the SDRAM provides
its own internal clocking, causing it to perform its own
AUTO REFRESH cycles. The SDRAM must remain in
self refresh mode for a minimum period equal to tRAS and
may remain in self refresh mode for an indefinite period
beyond that.
The procedure for exiting self refresh requires a sequence
of commands. First, CLK must be stable (stable clock is
defined as a signal cycling within timing constraints
AUTO PRECHARGE ensures that the precharge is initiated
at the earliest valid stage within a burst. The user must not
* Self refresh available in commercial and industrial temperatures only.
Ju;y 2006
Rev. 3
8
White Electronic Designs Corporation • (602) 437-1520 • www.wedc.com
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