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DDR2 series termination resistors

ASIM
Intellectual 270 points


Hi Forum, I am doing a 12 layer board which has 8 signal and 4 planes. PCB Stack is arranged such that every signal layer receives a plane on one side. I previously had routed DDR2 and DM6446 on a sixteen layer pcb, which went successful. In that older design i had 22 ohms series termination resistors between DDR2 and DM6446 chip.

I want to ask the forum: Can i remove those 22 ohms series termination reistors in my new 12 layer design if i make sure that the maximum track that runs between DM6446 and DDR2 chips is not more than an inch?

By removing these termination resistors i can reduce routing complexity as well as compact the placement of the DDR2 and DM6446 chips.

Please suggest,

Asim

  • 0
    Genius 4530 points

    Hi Asim,

    AS you must be knowing, series termination is generally recommended to avoid reflections on high speed tracks when the track length is large enough.

    So -

    1. Adding a series termination resistor depends upon the maximum speed at which you are going to operate the DDR in addition to the length of the track. Besides rise time and propagation delay of the associated signal is also important. If rise time of the signal is less than twice the propagation delay, then generally series termination resistor is necessary.

    2. You can also use DDR IBIS model to simulate the requirement of series termination resistor by adding the reistor and removing it and checking the ringing effect on the signal. This can give you a better idea of the requirement specific to your track length and DDR specifications.

    Regards,

    Sid

  • 0 in reply to Sid
    Intellectual 515 points

    Can we replace the Terminator Resistor by controlling the impedance of the trace?

    My PBC manufacturer has the ability of "impedance control" and that would be great to use because it does not require components (such as Resistors) and can be compact.