The procedure for exiting self refresh requires a sequence of commands. CK (and CK) must be stable prior to CKE returning high. Once CKE is high, the SDRAM must have NOP commands issued for tXSNR because time is required for the completion of any internal refresh in progress. A simple algorithm for meeting both refresh and DLL requirements is to apply NOPs for 200 clock cycles before applying any other command.
Operations:
Reads
Subsequent to programming the mode register with CAS latency, burst type, and burst length, Read bursts are initiated with a Read command.
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). The following timing figure entitled “Read Burst: CAS Latencies (Burst Length=4)” illustrates the 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 and 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 in timing figure entitled “Consecutive Read Bursts: CAS Latencies (Burst Length =4 or 8)”.A Read command can be initiated on any positive clock cycle following a previous Read command. Nonconsecutive Read data is shown in timing figure entitled “Non-Consecutive Read Bursts: CAS Latencies (Burst Length = 4)”. Full-speed Random Read Accesses: CAS Latencies (Burst Length = 2, 4 or 8) within a page (or pages) can be performed as shown on following:
CONFIDENTIAL – DO NOT COPY | Page 7-52 |
| File No. SG-0198 |
