2025-09-12 10:19 PM
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
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2025-09-13 1:32 AM - edited 2025-09-13 1:34 AM
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 ).
2025-09-13 1:32 AM - edited 2025-09-13 1:34 AM
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 ).