STM32H753 and SDRAM series resistor

[MFawzy]

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

[AScha.3]

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 ).