ESMT
M13S5121632A
Write Interrupted by a Precharge & DM
A burst write operation can be interrupted before completion of the burst by a precharge of the same bank. Random column
access is allowed. A write recovery time (tWR) is required from the last data to precharge command. When precharge command is
asserted, any residual data from the burst write cycle must be masked by DM.
<Burst Length = 8>
0
1
2
3
4
5
6
7
8
C L K
C L K
WRITE A
NO P
Precharge
NO P
NO P
WRITE B
NO P
NO P
NO P
C O M M A N D
t D Q S S m a x
DQ S
t W R
D i n a 6
D i n a 1 D i n a 2 D i n a 3 D i n a 4 D i n a 5
D i n a 7
D i n b 0
D i n a 0
D Q ' s
DQ S
t W R
t D Q S S m i n
D i n a 1 D i n a 2 D i n a 3 D i n a 4 D i n a 5
D i n a 7
D i n a 6
D i n b 1
D i n b 0
D i n a 0
D Q ' s
D M
Precharge timing for Write operations in DRAMs requires enough time to allow “Write recovery” which is the time required by a
DRAM core to properly store a full “0” or “1” level before a Precharge operation. For DDR SDRAM, a timing parameter, tWR, is used
to indicate the required of time between the last valid write operation and a Precharge command to the same bank.
The precharge timing for writes is a complex definition since the write data is sampled by the data strobe and the address is
sampled by the input clock. Inside the SDRAM, the data path is eventually synchronizes with the address path by switching clock
domains from the data strobe clock domain to the input clock domain.
This makes the definition of when a precharge operation can be initiated after a write very complex since the write recovery
parameter must reference only the clock domain that is used to time the internal write operation i.e., the input clock domain.
tWR starts on the rising clock edge after the last possible DQS edge that strobed in the last valid and ends on the rising clock
edge that strobes in the precharge command.
1. For the earliest possible Precharge command following a Write burst without interrupting the burst, the minimum time for write
recovery is defined by tWR
.
2. When a precharge command interrupts a Write burst operation, the data mask pin, DQ, is used to mask input data during the
time between the last valid write data and the rising clock edge in which the Precharge command is given. During this time, the
DQS input is still required to strobe in the state of DM.
The minimum time for write recovery is defined by tWR
.
3. For a Write with autoprecharge command, a new Bank Activate command may be issued to the same bank after tWR + tRP where
tWR + tRP starts on the falling DQS edge that strobed in the last valid data and ends on the rising clock edge that strobes in the
Bank Activate commands. During write with autoprecharge, the initiation of the internal precharge occurs at the same time as
the earliest possible external Precharge command without interrupting the Write burst as described in 1 above.
4. In all cases, a Precharge operation cannot be initiated unless tRAS(min) [minimum Bank Activate to Precharge time] has been
satisfied. This includes Write with autoprecharge commands where tRAS(min) must still be satisfied such that a Write with
autoprecharge command has the same timing as a Write command followed by the earliest possible Precharge command
which does not interrupt the burst.
Elite Semiconductor Memory Technology Inc.
Publication Date : Oct. 2008
Revision : 1.0 21/47
ESMT [ ELITE SEMICONDUCTOR MEMORY TECHNOLOGY INC. ]
