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MCB - RZQ vs Memory Device Attributes for DDR3

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Visitor
Posts: 10
Registered: ‎01-18-2011
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MCB - RZQ vs Memory Device Attributes for DDR3

I am confused by several statements in UG388 about RZQ and input/output impedance configuration in the MCB.

 

I feel that "Table 2-2: Memory Device Attributes" (UG388) describes the memory chip used. Whether it does or not, something is confusing about the 2 following attributes for a DDR3 device.

 

Memory Drive Strength
    DDR3: “DIV6“ (RZQ/6), “DIV7“ (RZQ/7)

Memory Termination Value (ODT)
    DDR3: “OFF“, “DIV2“ (RZQ/2), “DIV4“ (RZQ/4), “DIV6“ (RZQ/6), “DIV8“ (RZQ/8), “DIV12“ (RZQ/12)
    Note: RZQ = 240 Ω

 

As if they must satisfy some RZQ relationship.  That would greatly restrain the usable devices.

 

On page 47, it is stated:
"When Calibrated Input Termination is used, a resistor must be connected between the RZQ pin and ground with a value that is twice (2R) that of the desired input impedance (e.g., a 100Ω resistor to achieve a 50Ω effective input termination)."

 

So, RZQ is used to set the desired input impedance in the Spartan-6 but is also used to describe the device's ODT and driver strength. I can't find much values of RZQ that will satisfy all these equations, the real device caracteristics and our requirements for transmision line impedance matching.

 

In addition, what does "Note: RZQ 240Ω" bring to that description? Equation is valid only for RZQ=240? I guess not...

 

Can someone help me sort this out?

 

 


Accepted Solutions
Moderator
Posts: 4,131
Registered: ‎07-11-2011

Re: MCB - RZQ vs Memory Device Attributes for DDR3

[ Edited ]

Hi,

 

>> As if they must satisfy some RZQ relationship.  That would greatly restrain the usable devices\

-- Could you pleas eloborate ?

Options given in table 2-2 are as per JEDEC DDR2/3 Spec, please go through memory device  datasheet that you are using you can find the same sort of values there, MCB is just facilitating the spec.

Value of RZQ= 240+/-10% tolerance is also JEDEC Spec.

 

Page 47 mentions another RZQ and ZIO but that is for FPGAs input resistance, when you generate MIG you will see RZQ named pin in the ucf, you have connect a resistance = twice the desired impedance and the value will  be deicided by IBIS simulations

 

So RZQ applies to memory as well as FPGA, at memory value = 240Ohms, FPGA value will be based on your board physics and trace etc., impedance.

 

Please also refer this Xilinx AR  http://www.xilinx.com/support/answers/34055.html

  

Hope this helps

 

Regards,

Vanitha

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Moderator
Posts: 4,131
Registered: ‎07-11-2011

Re: MCB - RZQ vs Memory Device Attributes for DDR3

[ Edited ]

Hi,

 

>> As if they must satisfy some RZQ relationship.  That would greatly restrain the usable devices\

-- Could you pleas eloborate ?

Options given in table 2-2 are as per JEDEC DDR2/3 Spec, please go through memory device  datasheet that you are using you can find the same sort of values there, MCB is just facilitating the spec.

Value of RZQ= 240+/-10% tolerance is also JEDEC Spec.

 

Page 47 mentions another RZQ and ZIO but that is for FPGAs input resistance, when you generate MIG you will see RZQ named pin in the ucf, you have connect a resistance = twice the desired impedance and the value will  be deicided by IBIS simulations

 

So RZQ applies to memory as well as FPGA, at memory value = 240Ohms, FPGA value will be based on your board physics and trace etc., impedance.

 

Please also refer this Xilinx AR  http://www.xilinx.com/support/answers/34055.html

  

Hope this helps

 

Regards,

Vanitha

---------------------------------------------------------------------------------------------
Please do google search before posting, you may find relavant information.
Mark the post - "Accept as solution" and give kudos if information provided is helpful and reply oriented
Visitor
Posts: 10
Registered: ‎01-18-2011

Re: MCB - RZQ vs Memory Device Attributes for DDR3

I think I get it.  RZQ is used to describe two different resistors that can have different values, right?

 

Device's attributes most probably always apply to a 240 ohms RZQ (aka ZQ) since it is specified by Jedec.

 

MCB RZQ value as to be sized properly according to device settings and layout.

 

That's it?

 

 

Moderator
Posts: 4,131
Registered: ‎07-11-2011

Re: MCB - RZQ vs Memory Device Attributes for DDR3

correct!

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Visitor
Posts: 3
Registered: ‎09-06-2016

Re: MCB - RZQ vs Memory Device Attributes for DDR3

Sorry,  Some of us still do not get it.  This core generator explanation is unnecessarily obscure and seems self-contradictory.

 

Suppose I have a 50 ohm trace impedance for both input and output pins on the board hooked to one DDR3 SDRAM (not a module).  I allow for ODT on the data and data strobes. 

 

So if I presumably want ~25 ohms (series?) termination for the the drivers going out of the FPGA, and 50 ohms (pulldown?) for the signals coming back as inputs, what resistor should I put down for RZQ, and how should I answer each of these 3 questions.  Note that the Core Generator prompts suggest that this resistor MUST BE 240 ohms in one place and 100 ohms implied for this case in another.

 

1. Output driver impedance control.

2. RTT (nominal) - ODT

3a. Calibrated Input Termination

or

3b. UN-calibrated Input Termination