WEDPN16M64VR-XB2X
White Electronic Designs
a specified time (tRP) after the PRECHARGE command is
issued. InputA10 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.
ACTIVE
The ACTIVE command is used to open (or activate) a
row in a particular bank for a subsequent access. The
value on the BA0, BA1 inputs selects the bank, and
the address provided on inputs A0-A12 selects the row.
This row remains active (or open) for accesses until
a PRECHARGE command is issued to that bank. A
PRECHARGE command must be issued before opening
a different row in the same bank.
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 usingA10 to enableAUTO 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, whereAUTO PRECHARGE does
not apply. AUTO PRECHARGE is nonpersistent in that it
is either enabled or disabled for each individual READ or
WRITE command.
READ
The READ command is used to initiate a burst read
access to an active row. The value on the BA0, BA1 inputs
selects the bank, and the address provided on inputsA0-8
selects the starting column location. The value on input
A10 determines whether or not AUTO PRECHARGE is
used. If AUTO PRECHARGE is selected, the row being
accessed will be precharged at the end of the READ
burst; if AUTO PRECHARGE is not selected, the row will
remain open for subsequent accesses. Read data appears
on the I/Os subject to the logic level on the DQM inputs
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.
AUTO PRECHARGE ensures that the precharge is initiated
at the earliest valid stage within a burst. The user must
not 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
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 inputsA0-8
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.
BURST TERMINATE
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 256Mb SDRAM
requires 8,192 AUTO REFRESH cycles every refresh
period (tREF). Providing a distributed AUTO REFRESH
command will meet the refresh requirement and ensure
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
January 2005
Rev. 0
8
White Electronic Designs Corporation • (602) 437-1520 • www.wedc.com
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