I've been looking at schematics that interface ddr ram and noticed a weird notation for their zq pull down resistor. What is with the dot at the top as well as the 1%? Haven't seen this before.



Because these DDR's have merged drivers and on-chip termination they have an active termination calibration circuitry and procedure. The calibration resistor should be accurate , thus the 1% selection.

Note that this resistor can be shared amongst die if the controller sequences properly. Look at this app note from Micron (opens a PDF) concerning the calibration "DDR3 ZQ Calibration".

The dot ... see this EE.SE post

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From that data sheet.


1% is the resistor tolerance (its 'accuracy' value). The actual resistor used in the circuit must have a value that is within 1% of the specified 240R.
The dot may be an orientation indicator of some sort. Could this resistor be one part of a network or package of multiple resistors?


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