AS4C64M8D2
Bank activate command
The Bank Activate command is issued by holding CAS# and WE# HIGH with CS# and RAS# LOW at the rising
edge of the clock. The bank addresses BA0-BA1 are used to select the desired bank. The row addresses A0
through A13 are used to determine which row to activate in the selected bank. The Bank Activate command must
be applied before any Read or Write operation can be executed. Immediately after the bank active command, the
DDR2 SDRAM can accept a read or write command (with or without Auto-Precharge) on the following clock cycle. If
a R/W command is issued to a bank that has not satisfied the tRCDmin specification, then additive latency must be
programmed into the device to delay the R/W command which is internally issued to the device. The additive
latency value must be chosen to assure tRCDmin is satisfied. Additive latencies of 0, 1, 2, 3, 4, 5 and 6 are supported.
Once a bank has been activated it must be precharged before another Bank Activate command can be applied to
the same bank. The bank active and precharge times are defined as tRAS and tRP, respectively. The minimum time
interval between successive Bank Activate commands to the same bank is determined (tRC). The minimum time
interval between Bank Active commands is tRRD
Read and Write access modes
After a bank has been activated, a Read or Write cycle can be executed. This is accomplished by setting RAS#
HIGH, CS# and CAS# LOW at the clock’s rising edge. WE# must also be defined at this time to determine whether
the access cycle is a Read operation (WE# HIGH) or a Write operation (WE# LOW). The DDR2 SDRAM provides a
fast column access operation. A single Read or Write Command will initiate a serial Read or Write operation on
successive clock cycles. The boundary of the burst cycle is strictly restricted to specific segments of the page length.
Any system or application incorporating random access memory products should be properly designed, tested, and
qualified to ensure proper use or access of such memory products. Disproportionate, excessive, and/or repeated
access to a particular address or addresses may result in reduction of product life.
Posted CAS#
Posted CAS# operation is supported to make command and data bus efficient for sustainable bandwidths in DDR2
SDRAM. In this operation, the DDR2 SDRAM allows a CAS# Read or Write command to be issued immediately
after the RAS bank activate command (or any time during the RAS# -CAS#-delay time, tRCD, period). The command
is held for the time of the Additive Latency (AL) before it is issued inside the device. The Read Latency (RL) is
controlled by the sum of AL and the CAS latency (CL). Therefore if a user chooses to issue a R/W command before
the tRCDmin, then AL (greater than 0) must be written into the EMR(1). The Write Latency (WL) is always defined as
RL - 1 (Read Latency -1) where Read Latency is defined as the sum of additive latency plus CAS latency
(RL=AL+CL). Read or Write operations using AL allow seamless bursts (refer to seamless operation timing diagram
examples in Read burst and Write burst section)
Burst Mode Operation
Burst mode operation is used to provide a constant flow of data to memory locations (Write cycle), or from memory
locations (Read cycle). The parameters that define how the burst mode will operate are burst sequence and burst
length. The DDR2 SDRAM supports 4 bit and 8 bit burst modes only. For 8 bit burst mode, full interleave address
ordering is supported, however, sequential address ordering is nibble based for ease of implementation. The burst
length is programmable and defined by the addresses A0 ~ A2 of the MRS. The burst type, either sequential or
interleaved, is programmable and defined by the address bit 3 (A3) of the MRS. Seamless burst Read or Write
operations are supported. Interruption of a burst Read or Write operation is prohibited, when burst length = 4 is
programmed. For burst interruption of a Read or Write burst when burst length = 8 is used, see the “Burst
Interruption“ section of this datasheet. A Burst Stop command is not supported on DDR2 SDRAM devices.
Confidential
14
Rev. 1.0
Feb. /2014
ALSC [ ALLIANCE SEMICONDUCTOR CORPORATION ]
