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DDR3 VTT termination resistors and stub length

DJ McLeod
Intellectual 375 points

Hi,

Table 11 in section 4.3.1.4 in SPRABI1B lists the maximum stub length for the address / command lines to be < 80 mils. When designing with fly-by topology with two DDR3 chips running at 666 MHz, can the stub length be longer, ideally up to 300 mil? 80 mil seems impossible with this topology and the termination resistors.

Thanks,

DJ

  • 0
    TI__Mastermind 43660 points

    Hello DJ,

    The maximum stub length for the address / command lines to be < 80 mils is recommended across any speed of operation. Please note the recommended routing topology for address/command signals is fly-by and this stub length recommendation defined considering fly-by topology only.

    I believe this could be possible as most of our customers have implemented this in their design. You could minimize the via stub length by using blind/buried vias to achieve this limitation.

    Regards,
    Senthil

  • 0 in reply to Senthilkumar Srinivasan
    TI__Mastermind 43660 points
    Hello DJ,

    The address/command lines should have a stub length of <80 mils. To my knowledge, the stub length is nothing but the routing length from the second via to the memory chip and/or termination resistor.

    Considering your case, you can use the first via to switch the routing from top to bottom most layer and second via (two vias for two memory devices) from bottom most layer to top player. You have to take a tap from the second via (bottom most layer) used for second memory, route to some extend say around 250 mils and use a via for termination resistor. The via to resistor routing should be within 80 mils.

    The similar topology has been followed in the C6657 EVM as well. You could refer the C6657 EVM PCB layout for your reference.

    www.einfochips.com/.../C6657%20Lite%20EVM_PCB_17-00132-02.brd

    Regards,
    Senthil
  • 0 in reply to Senthilkumar Srinivasan
    Intellectual 375 points

    Thanks Senthil!

    Cheers,

    DJ