Datasheet

Table 14. Precharge & Auto Precharge Clariification
From Command
To Command
Minimum Delay between “From
Command” to “To Command”
Unit
Note
Read
Precharge (to same Bank as Read)
AL+BL/2+max(RTP,2)-2
t
CK
1,2
Precharge All
AL+BL/2+max(RTP,2)-2
Read w/AP
Precharge (to same Bank as Read w/AP)
AL+BL/2+max(RTP,2)-2
t
CK
1,2
Precharge All
AL+BL/2+max(RTP,2)-2
Write
Precharge (to same Bank as Write)
WL+BL/2+t
WR
t
CK
2
Precharge All
WL+BL/2+t
WR
Write w/AP
Precharge (to same Bank as Write w/AP)
WL+BL/2+t
WR
t
CK
2
Precharge All
WL+BL/2+t
WR
Precharge
Precharge (to same Bank as Precharge)
1
t
CK
2
Precharge All
1
Precharge All
Precharge
1
t
CK
2
Precharge All
1
NOTE 1: RTP [cycles] =RU {t
RTP
[ns]/t
CK
(avg) [ns]}, where RU stands for round up.
NOTE 2: For a given bank, the precharge period should be counted from the latest precharge command, either
one bank precharge or precharge all, issued to that bank.The prechrage period is satisfied after t
RP
or t
RP
all(=t
RP
for 8 bank device + 1X t
CK
) depending on the latest precharge command issued to that bank.
! Refresh command
When CS#, RAS# and CAS# are held LOW and WE# HIGH at the rising edge of the clock, the chip enters the
Refresh mode (REF). All banks of the DDR2 SDRAM must be precharged and idle for a minimum of the Precharge
time (t
RP
) before the Refresh command (REF) can be applied. An address counter, internal to the device, supplies
the bank address during the refresh cycle. No control of the external address bus is required once this cycle has
started.
When the refresh cycle has completed, all banks of the DDR2 SDRAM will be in the precharged (idle) state. A
delay between the Refresh command (REF) and the next Activate command or subsequent Refresh command
must be greater than or equal to the Refresh cycle time (t
RFC
).To allow for improved efficiency in scheduling and
switching between tasks, some flexibility in the absolute refresh interval is provided. A maximum of eight Refresh
commands can be posted to any given DDR2 SDRAM, meaning that the maximum absolute interval between any
Refresh command and the next Refresh command is 9 * t
REFI
.
! Self refresh operation
The Self Refresh command can be used to retain data in the DDR2 SDRAM, even if the rest of the system is
powered down. When in the Self Refresh mode, the DDR2 SDRAM retains data without external clocking. The
DDR2 SDRAM device has a built-in timer to accommodate Self Refresh operation. The Self Refresh Command is
defined by having CS#, RAS#, CAS# and CKE# held LOW with WE# HIGH at the rising edge of the clock. ODT
must be turned off before issuing Self Refresh command, by either driving ODT pin LOW or using EMRS
command. Once the Command is registered, CKE must be held LOW to keep the device in Self Refresh mode.
The DLL is automatically disabled upon entering Self Refresh and is automatically enabled upon exiting Self
Refresh. When the DDR2 SDRAM has entered Self Refresh mode all of the external signals except CKE, are
“don’t care”. For proper Self Refresh operation all power supply pins (V
DD
, V
DDQ
, V
DDL
and V
REF
) must be at valid
levels. The DRAM initiates a minimum of one refresh command internally within t
CKE
period once it enters Self
Refresh mode. The clock is internally disabled during Self Refresh Operation to save power. The minimum time
that the DDR2 SDRAM must remain in Self Refresh mode is t
CKE
. The user may change the external clock
frequency or halt the external clock one clock after Self Refresh entry is registered, however, the clock must be
restarted and stable before the device can exit Self Refresh operation.
The procedure for exiting Self Refresh requires a sequence of commands. First, the clock must be stable prior to
CKE going back HIGH. Once Self Refresh Exit is registered, a delay of at least t
XSNR
must be satisfied before a
valid command can be issued to the device to allow for any internal refresh in progress. CKE must remain HIGH
for the entire Self Refresh exit period t
XSRD
for proper operation except for Self Refresh re-entry. Upon exit from
Self Refresh, the DDR2 SDRAM can be put back into Self Refresh mode after waiting at least t
XSNR
period and
issuing one refresh command(refresh period of t
RFC
). NOP or Deselect commands must be registered on each
positive clock edge during the Self Refresh exit interval t
XSNR
. ODT should be turned off during t
XSRD
. The use of
Self Refresh mode introduces the possibility that an internally timed refresh event can be missed when CKE is
raised for exit from Self Refresh mode. Upon exit from Self Refresh, the DDR2 SDRAM requires a minimum of one
extra auto refresh command before it is put back into Self Refresh mode.
AS4C128M16D2A-25BCN
AS4C128M16D2A-25BIN
Confidential
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Rev.1.0 Dec 2015