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Altera_Forum's avatar
Altera_Forum
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12 years ago

Resitor Terminsion for LVDS ARRIA V

Hi all,

I'm designing a new board with LVDS connection:

On the receiver side, I implemented (on my quartus design) a deserialiser at 85MHz for a 7 serialise factor. I read on the device handbook that I do not need any 100Ohm resistor :

--- Quote Start ---

6-28 : The Arria V devices provide a 100 Ω, on-chip differential termination option on each differential receiver channel for LVDS standards. On-chip termination saves board space by eliminating the need to add external resistors on the board. You can enable on-chip termination in the Quartus II software Assignment Editor. All I/O pins and dedicated clock input pins support on-chip differential termination, RD OCT.

--- Quote End ---

Is it true for "simple" LVDS input (not transceiver input) ?

Do I need to care about RZQ pin ? I don't understand what is it function ? (I just copy-paste the DDR3 schematic from ALTERA dev Board and they use it, and I know that it is linked with impedance input calibration....)

On the transmitter side, do I need a network resistor (120-Ohm parallel and 170-Ohm in series) ?

Thanks in advance.

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    12 years ago

    For Arria V LVDS, there is indeed on chip termination resistor for RX. And if you are using ALTLVDS_RX and setup the input pin type as LVDS (of course), the 100 ohms termination resistor is on by default. In this case you don't need external termination resistor.

    But in real world design, we will put the on-board termination resistors (at least for me). In this case, either you can de-solder the on board resistors or you need to turn the Arria V OCT resistor off manually.

    here is the way to go in Quartus II assignment editor from the handbook, in order to turn it off, the value need to be set as "OFF".

    https://www.alteraforum.com/forum/attachment.php?attachmentid=7395

    It took me quite a while to figure this out since I noticed incorrect LVDS signal amplitude. In the handbook I found nothing about the default setting, on or off, for the LVDS OCT. Also the OCT type “Differential” together with other options such as "OCT 100" in pin assignment editor are confusing.

    To you questions, LVDS standard only require 1 termination resistor in receiving end. There is nothing for the transmitting part. you cannot use DDR3 as the template because it use other differential standard.
  • Altera_Forum's avatar
    Altera_Forum
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    12 years ago

    Hi sunxishan

    So I do not need to add resistor for LVDS receiver and transmitter.

    2 questions :

    - Why do you prefer to add the the 100 Ohms termination resistor and to either de-solder or turn off the OCT ?

    - Do I need to care about RZQ pin ?

    Thanks
  • Altera_Forum's avatar
    Altera_Forum
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    12 years ago

    actually I am not sure you fully understand my comments.

    1) you can add 100 ohms resistor on the PCB, in this case you need to turn off OCT. on the other hand if you design the board from very beginning, you can just forget all the termination resistors on PCB, which can save you PCB space and design work. you can choose one based on you project.

    2) RZQ is not existed when you use LVDS