EM68A16CBQC
EtronTech
z Mode Register Set (MRS)
The mode register stores the data for controlling the various operating modes of DDR2 SDRAM. It controls CAS
latency, burst length, burst sequence, test mode, DLL reset, WR, and various vendor specific options to make
DDR2 SDRAM useful for various applications.The default value of the mode register is not defined, therefore
the mode register must be programmed during initialization for proper operation. The mode register is written by
asserting LOW on CS#, RAS#, CAS#, WE#, BA0 and BA1, while controlling the state of address pins A0 - A12.
The DDR2 SDRAM should be in all bank precharge state with CKE already HIGH prior to writing into the mode
register.The mode register set command cycle time (tMRD) is required to complete the write operation to the
mode register. The mode register contents can be changed using the same command and clock cycle
requirements during normal operation as long as all bank are in the precharge state.The mode register is
divided into various fields depending on functionality.
- Burst Length Field (A2, A1, A0)
This field specifies the data length of column access and selects the Burst Length.
- Addressing Mode Select Field (A3)
The Addressing Mode can be Interleave Mode or Sequential Mode. Both Sequential Mode and Interleave
Mode support burst length of 4 and 8.
- CAS Latency Field (A6, A5, A4)
This field specifies the number of clock cycles from the assertion of the Read command to the first read data.
The minimum whole value of CAS Latency depends on the frequency of CK. The minimum whole value
≦
satisfying the following formula must be programmed into this field. tCAC(min)
CAS Latency X tCK
- Test Mode field: A7; DLL Reset Mode field: A8
These two bits must be programmed to "00" in normal operation.
- (BA0, BA1): Bank addresses to define MRS selection.
Table 5. Mode Register Bitmap
BA1 BA0 A12 A11 A10 A9
A8
A7
A6
A5
A4
A3
BT
A2
A1
A0 Address Field
0
0
PD
WR
DLL TM
CAS Latency
Burst Length
Mode Register
A8 DLL Reset
A7
0
Mode
A3
Burst Type
A2 A1 A0 BL
0
1
No
Normal
Test
0
1
Sequential
Interleave
0
0
1
1
0
1
4
8
Yes
1
A12 Active power down exit time Write recovery for autoprecharge*1
0
1
Fast exit (use tXARD
)
A11 A10
A9
0
WR(cycles)
A6 A5 A4
CAS Latency
Slow exit (use tXARDS
)
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
Reserved
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
Reserved
1
2
3
4
5
6
7
8
Reserved
BA1 BA0 MRS Mode
0
Reserved
0
0
1
1
0
1
0
1
MR
1
3
4
5
6
7
EMR(1)
EMR(2)
EMR(3)
0
1
0
1
Note 1:.For DDR2-667/800/1066, WR min is determined by tCK (avg) max and WR max is determined by tCK(avg) min. WR
[cycles] = RU {tWR[ns]/tCK(avg)[ns]}, where RU stands for round up. The mode register must be programmed to this
value.This is also used with tRP to determine tDAL
.
Rev. 1.0
9
May /2015
ETRON [ ETRON TECHNOLOGY, INC. ]
