DDR之ZQ .

What's the ZQ Calibration command?
it used to calibrate DRAM Ron & ODT values. In normal operation, the DDR3 SDRAM needs longer time to calibrate output driver and on-die termination circuits at initialization and relatively smaller time perform periodic calibrations.

There are two parameters exisited in the ZQ calibration commands. ZQCL and ZQCS.
 ZQCL is used to perform the initial calibration during power-up initialization sequences. other is used to perfor periodic calibrations to account for voltage and temperature variations.
ZQCL can be issued at anytime, it's up to the controller and the system enviroment. if the calibration finished, the calibrated values are transferred from the calibration engine to DRAM IO, which gets reflected a updated output driver and on-die termination values.

ZQCS, It  accurately correct the a minimum of 0.5%(ZQ correction) of RON and RTT impedance error withiin 64 nCK for all speed bins assuming the maximum sensitivities specified in the 'Output Driver Voltage and Temperature Sensitivity' and 'ODT Voltage and Temperature Sensitivity' tables. we can caculate it by the formula.
the spec give us a eg.
ZQcorrection /  [ ( Tsens * Tdriftrate) + (Vsens * Vdriftrate)]
if Tsens = 1.5% per degress, Vsens = 0.15% per MicroVoltagem, Tdriftrate = 1 degree per Sec and Vdriftrate = 15 microVoltage per sec. So the interval between ZQCS command is calcuated as : 0.5/ [(1.5 * 1) + (0.15 * 15)] = 0.133ms

ZQ pin needs external resistor, it should be tied throught a resister (240 ohm +/- 1%) to the ground. meanwhile, it can be share with another.

But spec told us that the total capacitive loading on the ZQ pin must be limited. We can see the table 59. so i won't list it on my blog. For detail things, advising you to reading the spec seriously. Everyone has his own understanding, if you have any question, please let me know, i'm glad to talk with u. that's mine.

The next chapter, I think i should review the initialization of the DDR3 SDRAM. there are much more diverse things in it. So it would be my next target.

 

ZQ用来校正DRAM的驱动能力Ron和ODT，以动态的加强DDR3的信号完整性，校验时考虑了温度，电压和SDRAM颗粒的制程差异