18
command is issued.
AUTO REFRESH
AUTO REFRESH is used during normal operation of the SDRAM and is analogous to CAS#-BEFORE-RAS# (CBR) REFRESH in
conventional DRAMs. This command is nonpersistent, so it must be issued each time a refresh is required. All active banks must
be PRECHARGED prior to issuing an AUTO REFRESH command. The AUTO REFRESH command should not be issued until the
minimum tRP has been met after the PRECHARGE command as shown in the Operations section.
The addressing is generated by the internal refresh controller. This makes the address bits “Don’t Care” during an AUTO REFRESH
command. The 512Mb SDRAM requires 8,192 AUTO REFRESH cycles every 32ms (tREF). Providing a distributed AUTO
REFRESH command every 3.9μs 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 32ms.
OPERATIONS
Bank/Row Activation
Before any READ or WRITE commands can be issued to a bank within the SDRAM, a row in that bank must be “opened.” This is
accomplished via the ACTIVE command which selects both the bank and the row to be activated (see Figure 6).
After opening a row (issuing an ACTIVE command), a READ or WRITE command may be issued to that row, subject to the tRCD
specification. tRCD (MIN) should be divided by the clock period and rounded up to the next whole number to determine the earliest
clock edge after the ACTIVE command on which a READ or WRITE command can be entered. For example, a tRCD specification of
20ns with a 125 MHz clock (8ns period) results in 2.5 clocks, rounded to 3. This is reflected in Figure 7, which covers any case where
2 < tRCD (MIN)/tCK ≤ 3 (the same procedure is used to convert other specification limits from time units to clock cycles). A
subsequent ACTIVE command to a different row in the same bank can only be issued after the previous active row has been “closed”
(precharged). The minimum time interval between successive ACTIVE commands to the same bank is defined by tRC.