NEC uPD72257, Preliminary User'S Manual

Page: 301 / 343

Setting MEMSCTRL.FSREFB = 0 forces the Memory Controller to refresh only
one row before putting the SDRAM into self-refresh mode.
MEMSCTRL.FSREFA does the same, except that it controls the refresh pattern
just after coming out of self-refresh mode.
Since it takes time between setting MEMSCTRL.SREF = 1 and entering self-
refresh mode, the Memory Controller provides the self-refresh status bit
MEMSCTRL.SREFSTAT to indicate that the SDRAM is already in self-refresh
mode.
If you want to disable the Memory Controller when the SDRAM is in self-refresh
mode, you should ensure that MEMSCTRL.SREFSTAT = 1 before disabling the
Memory Controller.
The self-refresh initiation time mainly depends on whether all rows or one row -
defined by MEMSCTRL.FSREFB - are to be refreshed before entering self-refresh
mode.
The SDRAM must remain in self-refresh mode for a minimum period, defined by
MEMSTMG0R.TRASMIN and can remain in self-refresh mode for an indefinite
period of time.
After the SDRAM exits self-refresh mode, the Memory Controller issues NOP
commands for the time defined by MEMSTMG0R.TXSR before it issues any other
command.
Caution
When the SDRAM is accessed while it is in self-refresh mode, the Memory
Controller returns undefined data in case of read and discards written data.
External Memory Interface Controller Chapter 9
Preliminary User's Manual S19203EE1V3UM00
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