HYB25D256[400/800/160]B[T/C](L)
256-Mbit Double Data Rate SDRAM
Functional Description
3.5.2
Reads
Subsequent to programming the mode register with CAS latency, burst type, and burst length, Read bursts are
initiated with a Read command, as shown on Figure 9 "Read Command" on Page 25.
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 that is accessed starts precharge
at the completion of the burst, provided tRAS has been satisfied. 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 is available following the CAS
latency after the Read command. Each subsequent data-out element is valid nominally at the next positive or
negative clock edge (i.e. at the next crossing of CK and CK). Figure 10 "Read Burst: CAS Latencies (Burst
Length = 4)" on Page 26 shows general timing for each supported 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 goes High-Z. Data from any Read burst may be concatenated
with or truncated with data from a subsequent Read command. In either case, a continuous flow of data can be
maintained. The first data element from the new burst follows 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 on Figure 11 "Consecutive Read Bursts: CAS Latencies (Burst
Length = 4 or 8)" on Page 27. A Read command can be initiated on any clock cycle following a previous Read
command. Nonconsecutive Read data is illustrated on Figure 12 "Non-Consecutive Read Bursts: CAS
Latencies (Burst Length = 4)" on Page 28. Full-speed Figure 13 "Random Read Accesses: CAS Latencies
(Burst Length = 2, 4 or 8)" on Page 29 within a page (or pages) can be performed as shown on Figure 13.
CK
CK
HIGH
CKE
CS
RAS
CAS
WE
x4: A0-A9, A11
x8: A0-A9
CA
x16: A0-A8
EN AP
A10
DIS AP
BA
CA = column address
BA = bank address
EN AP = enable Auto Precharge
DIS AP = disable Auto Precharge
BA0, BA1
Don’t Care
Figure 9
Read Command
Data Sheet
25
Rev. 1.21, 2004-07
02102004-TSR1-4ZWW
QIMONDA [ QIMONDA AG ]
