HYB25D512800BT Datasheet, PDF (31/90 Pages) Infineon Technologies AG – 512Mbit Double Data Rate SDRAM

HYB25D512[40/16/80]0B[E/F/C/T]
512Mbit 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.
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 t_RAS 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 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 post
amble. 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 pre fetch architecture). This is shown on Figure 11. A
Read command can be initiated on any clock cycle following a previous Read command. Nonconsecutive Read
data is illustrated on Figure 12. Full-speed Random Read Accesses: CAS Latencies (Burst Length = 2, 4 or 8)
within a page (or pages) can be performed as shown on Figure 13.
Data from any Read burst may be truncated with a Burst Terminate command, as shown on Figure 14. The Burst
Terminate latency is equal to the read (CAS) latency, i.e. the Burst Terminate command should be issued x cycles
after the Read command, where x equals the number of desired data element pairs.
Data from any Read burst must be completed or truncated before a subsequent Write command can be issued. If
truncation is necessary, the Burst Terminate command must be used, as shown on Figure 15. The example is
shown for t_DQSS(min). The t_DQSS(max) case, not shown here, has a longer bus idle time. t_DQSS(min) and t_DQSS(max) are
defined in Chapter 3.5.3.
A Read burst may be followed by, or truncated with, a Precharge command to the same bank (provided that Auto
Precharge was not activated). The Precharge command should be issued x cycles after the Read command,
where x equals the number of desired data element pairs (pairs are required by the 2n prefect architecture). This
is shown on Figure 16 for Read latencies of 2 and 2.5. Following the Precharge command, a subsequent
command to the same bank cannot be issued until t_RP is met. Note that part of the row precharge time is hidden
during the access of the last data elements.
In the case of a Read being executed to completion, a Precharge command issued at the optimum time (as
described above) provides the same operation that would result from the same Read burst with Auto Precharge
enabled. The disadvantage of the Precharge command is that it requires that the command and address busses
be available at the appropriate time to issue the command. The advantage of the Precharge command is that it
can be used to truncate bursts.
Data Sheet
31
Rev. 1.2, 2004-06