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SRAM

Anonymous
Not applicable

Hi all,

I'm now designing the PCB with CY7C25652KV18 and XILINX FPGA. I'd like to set the trace impedance of the line between SRAM and FPGA.

According to the data sheet, the output impedance of the SRAM's output pin can be set by using RQ resistor between ZQ pin and the ground. The data sheet says that the output impedance shall be set to 0.2 x RQ. So I set the RQ to 250ohm in order to get the 50ohm trace impedance.

Then the input impedance of some SRAM's input pins are defined by On Die Termination function. In this case the input impedance shall be set to RQ/3.33 or RQ/1.66 according to data sheet. For both case, the input impedance cannot be set to 50ohm if RQ = 250ohm. In other words, it is impossible to have same impedance value for the input and output impedance of the SRAM pins. I found the similar discussion in this forum. That is On-Die Termination ZQ value?

On the other hand, there is the description "Cypress Packages are routed to obtain all traces to 50 ohm ± 10%. All traces must be routed to have 50-Ω impedance and should have no impedance discontinuities." in "AN4065 - QDR®-II, QDR-II+, DDR-II, and DDR-II+ Design Guide" It says that the input and output impedance of the SRAM is the same 50ohm.

Therefore I'm now much confused. Which is correct, AN4065 or the content of the discussion? Or do I misunderstand anything?

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1 Solution
Moderator
Moderator

Hi

Yes you are correct.

Externally, we cannot have same input and output impedance on this device and internally Cypress Packages are routed to obtain all traces to 50 ohm. We have recommendation in AN4065 (Page# 24). We would suggest you to use 250ohm RQ option.

pastedImage_1.png

Thanks,

Pradipta.

View solution in original post

3 Replies
Moderator
Moderator

Hi

Yes you are correct.

Externally, we cannot have same input and output impedance on this device and internally Cypress Packages are routed to obtain all traces to 50 ohm. We have recommendation in AN4065 (Page# 24). We would suggest you to use 250ohm RQ option.

pastedImage_1.png

Thanks,

Pradipta.

View solution in original post

Anonymous
Not applicable

Dear Pradipta,

Thank you for your suggest. Could I ask you one more question? Is it possible to set the impedance of three kinds of pins(outputs, inputs w/ ODT, and inputs w/o ODT) to the different value as below?

  1. SRAM's output impedance of Qn, CQ, /CQ to 40ohm(=RQx0.2, RQ=200ohm)
  2. SRAM's input impedance of Dn, /BWSn, K, /K (w/ ODT) to 60ohm(=RQ/3.33, RQ=200ohm)
  3. SRAM's input impedance of An, /RPS, /WPS (w/o ODT) to 50ohm(by 50ohm pull-up to VTT)

Regards,

Akihiko

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Moderator
Moderator

Hi Akihiko,

Yes, it is possible if you set the RQ= 200 ohms, as you mentioned as we have the acceptable range from 175 ohm to 350 ohm. Whichever combination gives you the optimum performance for your application you can go with that. Please note that as the output impedance is 40 ohm you will need to connect a 10 ohm resistor to match it close to 50 for better performance. If you do not connect this you will observe noisy output signals.

Thanks and Regards,

Pradipta.