Datasheet
Fine Granularity Refresh Mode
DDR4 supports Command Address Latency (CAL) function as a power savings feature. CAL is the delay in
clock cycles between CS# and CMD/ADDR defined by MR4[A8:A6].
CAL gives the DRAM time to enable the CMD/ADDR receivers before a command is issued. Once the
command and the address are latched, the receivers can be disabled. For consecutive commands, the DRAM
will keep the receivers enabled for the duration of the command sequence.
Mode Register and Command Truth Table
The Refresh cycle time (t
RFC
) and the average Refresh interval (t
REFI
) can be programmed by the MRS
command. The appropriate setting in the mode register will set a single set of Refresh cycle time and average
Refresh interval for the device (fixed mode), or allow the dynamic selection of one of two sets of Refresh cycle
time and average Refresh interval for the device (on-the-fly mode). The on-the-fly (OTF) mode must be
enabled by MRS before any on-the-fly Refresh command can be issued.
Table 17. MR3 definition for Fine Granularity Refresh Mode
A8
A7
A6
Fine Granularity Refresh
0
0
0
Normal Mode (Fixed 1x)
0
0
1
Fixed 2x
0
1
0
Fixed 4x
0
1
1
Reserved
1
0
0
Reserved
1
0
1
Enable on the fly 2x
1
1
0
Enable on the fly 4x
1
1
1
Reserved
There are two types of on-the-fly modes (1x/2x and 1x/4x modes) that are selectable by programming the
appropriate values into the mode register. When either of the two on-the-fly modes is selected (‘A8=1’), the
device evaluates BG0 bit when a Refresh command is issued, and depending on the status of BG0, it
dynamically switches its internal Refresh configuration between 1x and 2x (or 1x and 4x) modes, and
executes the corresponding Refresh operation.
Table 18. Refresh command truth table
Function
CS#
ACT#
RAS#/
A16
CAS#/
A15
WE#/
A14
BG0
BA0-1
A10/AP
A[9:0],
A[13:11]
MR3
[8:6]
Refresh (Fixed rate)
L
H
L
L
H
V
V
V
V
0VV
Refresh (on-the-fly 1x)
L
H
L
L
H
L
V
V
V
1VV
Refresh (on-the-fly 2x)
L
H
L
L
H
H
V
V
V
101
Refresh (on-the-fly 4x)
L
H
L
L
H
H
V
V
V
110
AS4C256M16D4
Confidential
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Rev.1.0 Aug.2019