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.
Does thermal information change for IT parts?
Thermal information includes temperature limits and thermal impedance values. Temperature limits do change for IT parts (TC, TJ, and TA), but thermal impedance values (θJA, θJB, and θJC) do not because thermal impedance depends primarily on the package.
My design was based on a specification stating the JTAG was relative to VDD (1.8V), but now we’ve discovered that JTAG is actually relative to VDDQ (1.5V). It’s a fairly significant board spin to change this; what do I risk by leaving the design as-is? I assume that the specification is still for VDDQ + 0.3V = 1.8V, but with CMOS parts there’s no way I can guarantee that it won’t swing past that on transitions.
Your particular board design should not be a cause of major concern. The pins can handle the VDD voltage regardless of the VDDQ voltage.
Should the ECC memory chip share chip select and CKE signals with the other two main memory chips in our point-to-point application?
The ECC chip(s) should share the same CKE and CS# as the other devices because they are accessed as the same piece of data.
What is a "bank"?
A bank is an array of memory bits. Multiple arrays or banks are contained within a DRAM component. Depending on density, DRAM components may consist of 4 or 8 banks. For example, a bank may consist of 32 million rows, 4 bits across. This would equate to 128 megabits. Four of these banks in a single DRAM component would yield a 512Mb component.
What is the impedance tolerance of the driver in match-impedance mode relative to the expected value base on the perfect reference resistor connected to ZQ pin?
The impedance tolerance of the driver is ±15 percent.