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

[satish2projects]

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.

[robertferanec]

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"