VG37648041AT datasheet(21/86 Pages) VML

21 / 86 page

Document : 1G5-0157

Rev.1

Page 21

VIS

Preliminary

VG37648041AT

256M:x4, x8, x16

CMOS Synchronous Dynamic RAM

DON’T CARE

CLK#

CLK

COMMAND

A0-A12

ACT

NOP

ACT

Figure 5

tRCD AND tRRD Definition

NOP

RD/

NOP

Row

Col

BA0,BA1

Row

Bank x

Bank y

tRRD

tRCD

READs

READ bursts are initiated with a READ command, as shown in Figure 6.

The starting column and bank addresses are provided with the READ command and AUTO PRECHARGE is

either enabled or disabled for that burst access. If AUTO PRECHARGE is enabled, the row being accessed is pre-

charged at the completion of the burst. For the generic READ commands used in the following illustrations, AUTO

PRECHARGE is disabled.

During READ bursts, the valid data-out element from the starting column address will be available following the

CAS latency after the READ command. Each subsequent data-out element will be valid nominally at the next pos-

itive or negative clock edge (i.e. at the next crossing of CLK and CLK#). Figure 7 shows general timing for each

posible CAS latency setting. DQS is driven by the DDR SDRAM along with output data. The initial LOW state on

DQS is known as the read preamble; the LOW state coincident with the last data-out element is known as the read

postamble.

Upon completion of a burst, assuming no other commands have been initiated, the DQs will go High-Z.

Data fom any READ burst may be concatenated with or truncated with data from a subsequent READ com-

mand. In either case, a continuous flow of data can be maintained. The first data element from the new burst fol-

low either the last element of a completed burst or the last desired data element of a longer burst which is being

truncated. The new READ command should be issued x cycles after the first READ command, where x equals the

number of desired data element pairs (pairs are required by the 2n prefetch architecture). This is shown in Figure

8. A READ command can be initiated on any clock cycle following a previous READ command. Non consecutive

READ data is shown for illustration in figure 9. Full-speed random read accesses within a page (or pages) can be

performed as shown in Figure 10.

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