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While manually configuring a Memory Interface Generator in Vivado, I saw an option related to the output driver impedance. There were two options selectable, RZQ/6 and RZQ/7.

I searched for it and find some citations in a document. It is generally mentioned as external resistor or external precision resistor.

So, what does it really stands for? What is the purpose of this resistor? What is the expanded form of the RZQ?

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2 Answers 2

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The Digilent web page for your Arty board (found in your previous question) says the correct settings and it is RZQ/6.

The RZQ means a 240 ohm reference impedace for the drive strength for the data bus, and DDR3 chips can for example use RZQ/6 for 40 ohms or RZQ/7 for 34 ohms.

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  • \$\begingroup\$ It is also cited on the thirty-ninth page of the MIG user guide UG586. \$\endgroup\$ Commented Sep 13, 2021 at 18:33
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On p.6 of the same document, it says:

To calibrate output driver impedance, an external precision resistor, RZQ, is connected between the ZQ pin and VSSQ. The value of this resistor must be 240 Ω ± 1%. If you are using a DDR3 SDRAM DIMM, RZQ is soldered on the DIMM so you do not need to layout your board to account for it.

So, the term "RZQ" is just an arbitrary name for the resistor. And the resistor's purpose is well explained.

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