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STM32H753 and SDRAM series resistor

MFawzy
Associate III

Hi,

I’ve seen several discussions about the use of series resistors between the STM32H753 and external SDRAM, and I’d like to get some clarification. I’m currently building a prototype of my custom board and noticed that on the STM32H753I-EVAL board, 33 Ω resistors are used on the SDRAM interface, while in other designs 0 Ω links are used instead.

For a first prototype, would you recommend starting with 33 Ω resistors (as on the eval board), or starting with 0 Ω and only switching to 33 Ω if I encounter signal integrity issues?

Thanks,
Mohammed

1 ACCEPTED SOLUTION

Accepted Solutions
AScha.3
Super User

Hi,

>would you recommend starting with 33 Ω resistors

 

short:  yes. Try 33 or 51 r .

+

long: the resistors are for reducing ringing and reflections on a wire/track .

So it depends on the source-line-destination-impedance and the signal speed (rise/fall times), whether you need line damping resistors, or not. And track length (all equal in length ! ) of course.

In general for hi-speed signals its always good idea to put impedance controlling/damping resistors in every track.

Also they are not "for free", so if the cost of some resistors is a problem...they might be omitted.

And the optimum value you have to guess, or calculate, from impedance of track (micro strip, ground plane -> see tools for impedance calculation) and impedance of involved chips (see ds ...if nothing else possible, can guess the driver impedance from max. current output ).

If you feel a post has answered your question, please click "Accept as Solution".

View solution in original post

1 REPLY 1
AScha.3
Super User

Hi,

>would you recommend starting with 33 Ω resistors

 

short:  yes. Try 33 or 51 r .

+

long: the resistors are for reducing ringing and reflections on a wire/track .

So it depends on the source-line-destination-impedance and the signal speed (rise/fall times), whether you need line damping resistors, or not. And track length (all equal in length ! ) of course.

In general for hi-speed signals its always good idea to put impedance controlling/damping resistors in every track.

Also they are not "for free", so if the cost of some resistors is a problem...they might be omitted.

And the optimum value you have to guess, or calculate, from impedance of track (micro strip, ground plane -> see tools for impedance calculation) and impedance of involved chips (see ds ...if nothing else possible, can guess the driver impedance from max. current output ).

If you feel a post has answered your question, please click "Accept as Solution".