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What is ZQ calibration - In DDR3, there are 2 different calibration commands to account for variations in the system environment for temperature, voltage, component drift.  The ZQ calibration commands calibrate the DRAM's output drivers (Ron) and ODT values (Rtt), normally, using a precision 240 ohm (±1 percent) resistor connected from the DRAM's ZQ pin to ground.

Since the first ZQ calibration long (ZQCL) command issued takes 512 clocks to complete, it is often used during power-up initialization and reset. Subsequent ZQCL commands can be issued at any time the DRAM is idle and will only require 246 clocks.

The ZQ calibration short (ZQCS) requires  64 clocks to complete so it is used periodically when the DRAM is idle to perform calibrations to account for minor variations in voltage and temperature. Each ZQCS command can correct a minimum of 0.5% impedance error within 64 clocks. As a starting point for BIOS development, you can determine the time interval between ZQCS commands, by calculating the ZQCS time interval as follows:

ZQCS time Interval =  ZQCorrection of  0.5%/64 clocks  /  [(Tsensitivities x Tdriftrate) + (Vsensitivities x Vdriftrate)]

Use the Maximum values from the ODT temperature and voltage sensitivity table shown in the DRAM component specification/datasheet, but the driftrates are system dependant and will vary system environments (often determined by testing in an environmental lab).

Both ZQCL and ZQCS can be initiated at any time and often as required by the BIOS and memory controller to account for larger changes in the system environment. The ZQCL command is used when there is more impedance error correction required than a ZQCS can provide.



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