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I am trying to figure out whether I need termination resistors for my PCB routing between a µ-Controller and an SDRAM (IS42S32800J-6BLI) that I plan to clock at 166 MHz. According to Table 72 of ST's datasheet for the STM32H745, the rise/fall times are 1.5 ns at a supply voltage of 3.3 V, 10 pF load, and maximum speed setting.

If I follow the guidance of the last answer of this stack exchange post, I get a bandwidth of BW = 0.35/t_rise = 233 MHz. An eighth of the wave length amounts to 16.1 cm.

I have done some first routing and expect my trace lengths to be no more than 4 cm. So it appears to me that I don't need any termination resistors. Yet, when I look at some of ST's evaluation board schematics (like this one for the STM32H745 discovery board), I see that they use 33 Ω resistors. I checked their board routing and while they are longer than what I expected for my PCB, they are nowhere near the 16.1 cm. Am I ill-advised by leaving them out? What would be the reasons that I should add termination resistors?

For completeness sake, I've done some microstrip calculations using this online calculator and ended up with a single-ended impedance of 113 Ω for a top or bottom layer route. The propagation delay is 54 ps/cm.

Please give me some guidance on how to proceed here. Is there something that I am missing? Ideally I want to avoid adding 44 resistors as they will occupy a lot of space and make routing harder on a 4-layer PCB, even if the resistors are just size 0402.

Here's a summary of the calculations:

enter image description here

Hansel
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  • Are you absolutely 100% sure that they are termination resistors? On the face of it they look like series source termination resistors but, when the data is outputted from the SDRAM, they are at the "wrong end" to act as a termination resistor. You need to look into the ST design. Maybe input ports and output ports are on different pins - I haven't looked in that much detail so I can't say. `a single-ended impedance of 113 Ω` seems a little high to me. – Andy aka Dec 11 '21 at 11:32
  • Do you need to comply with any EMC requirements? It could be the resistors are to soften the edges a little so the board doesn’t radiate as much. Or you could put a can over the circuit. – Kartman Dec 11 '21 at 11:49
  • @Andyaka (I can't get the quoting to work), I can't say I am absolutely sure these are termination resistors and you have a point as the address resistors should be placed near the µC and for the data signals they should be placed near the SDRAM. The 113 Ω are calculated, though I must admit I have seen typical values float around on the internet that are closer to 50...70 Ω. – Hansel Dec 11 '21 at 12:08
  • @Kartman, no, I don't see the need to adhere to EMC. I will have some analog part on the board though but I am not too concerned about it as I will be operating in the audio range. I'm fairly familiar with separating things, avoiding ground loops, current return paths, etc. – Hansel Dec 11 '21 at 12:11
  • @Hansel regards quoting try this ` `text` ` - remember that the data lines are bidirectional and, a series termination resistor doesn't help stop reflections arising if it's placed at the "wrong" end - which end do you put termination resistors when the line is bidirectional I wonder? Maybe the lines are not bidirectional - can you confirm? Address lines are OK - terms need to be at the source end of course. Thinking more, for bidirectional lines to be effectively terminated, a series resistor should be placed at both ends - yes, having thought about this, that is correct. – Andy aka Dec 11 '21 at 13:34
  • @Andyaka, yes, you are right, of course the data lines are bidirectional, so my comment that the resistors should be placed near the SDRAM is incorrect. I rechecked the schematic of the evaluation board and I see only one resistor per data line. So I get more and more the impression that the purpose of the resistors is indeed just for EMC. – Hansel Dec 11 '21 at 15:06
  • That is indeed what I was thinking @Hansel - maybe you can find some write-up about the ST design - I mean you might find something that says they are series termination resistors then, you know that maybe the ST board is not a good reference if the reason given by the designer is series term (because they can't be). Dig deeper is my advice but, the jury is swaying towards EMI reduction (from possibly the faster CPU?). – Andy aka Dec 11 '21 at 15:13
  • Thanks @Andyaka. So, then, is the verdict that I don't really need any termination? Or would it be advisable for reasons unrelated to EMC/EMI? – Hansel Dec 11 '21 at 16:21
  • I think you need to dig deeper and double check things. You have at least one opinion (mine) but, that's not enough for me at this point. – Andy aka Dec 11 '21 at 16:28

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