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Application Note With Impedance of 75 Ohms. Which to choose 50 or 75 Ohms?.

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  • Application Note With Impedance of 75 Ohms. Which to choose 50 or 75 Ohms?.

    Dear Sir,
    And another doubt is, what is the Rise Time And Fall Time of Processor for ATSAM9G20.
    They have not mentioned any Timings in Data-sheet. Where to get all this Information for Other Processor ARM-Cortex-A8 too. I have analyzed PCB Stack-Up in ICD Stack-Up Planner for 8-Layer Board. In application note page no:34 of ATSAM they have mentioned 75 Ohm Impedance for Signal and 15 Ohms for Planes but in ICD Stack-Up Planner I have Set to 50 Ohm Impedance +/- 5 % Tolerance Impedance. Which one should I follow?. I am bit confused. I am using BGA Package.
    For DDR SDRAM is Length matching is required?. In some blogs in Internet I have seen that length matching is not required for DDR SDRAM is it Correct?.
    Sir Kindly reply in this matter as soon as possible. I will be waiting most eagerly for your precious reply.

  • #2
    satish2projects I have just answered your original question here.

    I had a brief look into datasheet and rise time seems to me to be explained at page 11 - depends on voltage used for the pins.

    You can set some parameters on the processor (e.g. pull up / down) and that influences impedance. Have a look at page 18: "As seen, by comparing the buffer output impedance value (5 Ohms up to 10 Ohms, approximately) and the typical characteristic impedance (50 Ohms up to 100 Ohms), adding a termination resistor can be anticipated"