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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 SDR SDRAM data sheets require that the clock frequency be constant during access or precharge states. However, because there is no DLL in SDRAM, it may be possible to shift the clock frequency dynamically, though this is not recommended by Micron. If a design requires shifting frequency, lowering SDRAM frequency may be 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 must always be adhered to.
Is there a minimum clock frequency for SDR SDRAM?

Because SDR SDRAM does not have a DLL, there is no minimum clock frequency.  However, if the device is clocked at lower frequencies, it is still important to maintain a reasonably fast slew rate on the clock edges to avoid risk of setup and/or hold-time violations. Also, for operating frequencies of 45 MHz, tCKS = 3.0ns. For more information, see LVTTL Derating for SDRAM Slew Rate Violations (TN-48-09).