Datasheet

Programming the Mode Registers
For application flexibility, various functions, features, and modes are programmable in seven Mode Registers,
provided by the DDR4 SDRAM, as user defined variables and they must be programmed via a Mode Register
Set (MRS) command. The mode registers are divided into various fields depending on the functionality and/or
modes. As not all the Mode Registers (MRn) have default values defined, contents of Mode Registers must be
initialized and/or re-initialized, i.e. written, after power up and/or reset for proper operation. Also the contents of
the Mode Registers can be altered by re-executing the MRS command during normal operation. When
programming the mode registers, even if the user chooses to modify only a sub-set of the MRS fields, all
address fields within the accessed mode register must be redefined when the MRS command is issued. MRS
command and DLL Reset do not affect array contents, which means these commands can be executed any
time after power-up without affecting the array contents. MRS Commands can be issued only when DRAM is
at idle state. The mode register set command cycle time, t
MRD
is required to complete the write operation to the
mode register and is the minimum time required between two MRS commands shown in the t
MRD
timing figure.
T1 T2 Ta0 Ta1 Ta2 Tb0 Tb1 Tb2 Tb3T0 Tb4
t
MRD
VALIDVALID VALID MRS
2
DES DES DES MRS DES DES DES
VALIDVALID VALID VALID VALID VALID VALID VALID VALID VALID VALID
CK#
ADDR
CK
CMD
CKE
Updating Settings
NOTE 1. This timing diagram shows C/A Parity Latency mode is “Disable” case.
NOTE 2. List of MRS commands exception that do not apply to t
MRD
- Gear down mode
- C/A Parity Latency mode
- CS to Command/Address Latency mode
- Per DRAM Addressability mode
- V
REFDQ
training Value, V
REFDQ
Training mode and V
REFDQ
training Range.
Old Settings
Settings
DON'T CARETIME BREAK
Figure 6. t
MRD
timing
Some of the Mode Register setting affect to address/command/control input functionality. These case, next
MRS command can be allowed when the function updating by current MRS command completed.
The MRS commands that do not apply t
MRD
timing to next MRS command. These MRS command input
cases have unique MR setting procedure, so refer to individual function description.
AS4C256M16D4
Confidential
- 13 of 201 -
Rev.1.0 Aug.2019