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Can CKE be tied HIGH throughout SDRAM operation (initialization and normal operation)?
JEDEC does not specify the exact state of CKE during initialization; it is supplier specific. Micron strongly recommends CKE be kept at an LVTTL logic LOW before applying a stable CLK signal. During normal operation, CKE can be tied HIGH. The initial LOW state of CKE prevents parts from receiving an illegal LMR command, which could put the part into an unknown or unexpected state.
Can the SDRAM clock frequency be changed?
Micron SDRAM data sheets require that the clock frequency be constant during access or precharge states (READ, WRITE, tWR, and PRECHARGE commands). At other times frequency should not matter much because there is no DLL in SDRAM however, we do not recommend it. Lowering SDRAM frequency is OK even if you are not doing an LMR and CAS latency change. In case of increasing frequency, ensure tCK and CAS latency specifications are met. In either case, all other data sheet timing specifications should be adhered to.
Is there a recommended lowest working frequency for SDRAM?
Because SDRAM does not have a DLL, there is no recommended lowest frequency. SDRAM parts will work at very low frequencies if all data sheet specifications are met. It is important to maintain a good slew rate, however, since a very slow slew rate will affect setup and hold-time transitions. Also, for operating frequencies of 45 MHz, tCKS = 3.0ns. For more information, see TN-48-09.