13

Rev. 0A | October 2022

www.issi.com - dram@issi.com

IS43/46LR32320C

IS43/46LR16640C

Mode Register

The mode register is used to define the specific mode of operation of the Mobile DDR SDRAM. This definition includes the selection of a

burst length, a burst type, a CAS latency. The mode register is programmed via the LOAD MODE REGISTER command and will retain the

stored information until programmed again, the device goes into deep power-down mode, or the device loses power.

Mode register bits A0-A2 specify the burst length, A3 specifies the type of burst (sequential or interleaved), A4-A6 specify the CAS latency,

and A7-A12(A13) should be set to zero. BA0 and BA1 must be zero to access the mode register.

The mode register must be loaded when all banks are idle, and the controller must wait the specified time before initiating the subsequent

operation. Violating either of these requirements will result in unspecified operation.

Burst Length

Read and write accesses to the Mobile DDR SDRAM are burst oriented, with the burst length being programmable, as shown in Figure

(Mode Register Set Definition). The burst length determines the maximum number of column locations that can be accessed for a given

READ or WRITE command. Burst lengths of 2, 4, or 8 are available for both the sequential and the interleaved burst types.

Reserved states should not be used, as unknown operation or incompatibility with future versions may result. When a READ or WRITE

command is issued, a block of columns equal to the burst length is effectively selected. All accesses for that burst take place within this

block, meaning that the burst will wrap within the block if a boundary is reached. The block is uniquely selected by A1-A9 when the burst

length is set to two; by A2-A9 when the burst length is set to four; by A3-A9 when the burst length is set to eight; and by A4-A9 when the

burst length is set to sixteen. The remaining (least significant) address bit(s) is (are) used to select the starting location within the block.

The programmed burst length applies to both READ and WRITE bursts.

CAS Latency

The CAS latency is the delay, in clock cycles, between the registration of a READ command and the availability of the first bit of output

data. The latency can be set to 2, 3 clocks, as shown in Figure (Standard Mode Register Definition).

For CL = 3, if the READ command is registered at clock edge n, then the data will be available at (n + 2 clocks + tAC). For CL = 2, if the

READ command is registered at clock edge n, then the data will be available at (n + 1 clock + tAC).

Figure7 : CAS Latency (BL=4)

/C K

C K

Command

T0

T1

T2

T3

T1n

T2n

T3n

READ

NOP

NOP

NOP

DQS

DQ

tAC

CL=3

DOUT

n+1

tRPRE

2tCK

T4

T4n

NOP

tRPST

DOUT

n

DOUT

n+2

DOUT

n+3

Don’t care

DQS

DQ

tAC

CL=2

DOUT

n+1

tRPRE

1tCK

tRPST

DOUT

n

DOUT

n+2

DOUT

n+3

L

L