NEC uPD72257, Preliminary User'S Manual

Page: 300 / 343

9.2.2.2 Self-refresh mode
You can put the SDRAM into self-refresh mode, at which point the SDRAM retains
data without external clocking and auto-refresh. Below figure illustrates the
command sequence issued by the SDRAM controller to initiate, maintain, and exit
the self-refresh mode.
Exit
self-refresh
mode
Refresh
before
self-refresh
Refresh
after
self-refresh
Enter
self-refresh
mode
Ready for data transfer
MEMSCTRL.SREF = 1
MEMSCTRL.SREF = 0
Self-refresh mode
NOP
NOP
NOP
Auto-refresh
single (FSREFA=0)
or all (FSREFA=1) rows
Auto-refresh
single (FSREFB=0)
or all (FSREFB=1) rows
MEMSTMG0R.
TXSR
MEMSTMG0R.
TRASMIN
Figure 9-5 Self-refresh command sequence
You can force the Memory Controller to enter self-refresh mode by setting
MEMSCTRL.SREF = 1.
The Memory Controller forces the SDRAM to come out of self-refresh mode when
setting MEMSCTRL.SREF = 0.
The control bits FSREFB and FSREFA of the MEMSCTRL register specify the type
of refresh done by the Memory Controller just prior to entering self-refresh mode
and just after leaving self-refresh mode.
Chapter 9 External Memory Interface Controller
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Preliminary User's Manual S19203EE1V3UM00