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Why are series resistance used with External Ram with STM32H750-DK


I.want to know the use of series resistance that is placed with STM32H750B-DK and External RAM , And also.with it's Dual Quad SPI interface.

Is it possible to leave them as it is or it is compulsory to implement.

David Littell
Senior III

They're there for a little thing called "signal integrity".  Real EE's can explain why they are indeed mandatory in high-quality designs.

Dear @David Littell , @ZExpo.1 

thanks for the contribution, indeed when running high speed signals - these serial resistors become Your friends and save you a lot of debugging if well designed at PCB level . The main goal is to keep impedance matching from the Driver /receiver  and the transmission line - trace paths and so avoid signal reflections and ringing that will create shadow signals affecting overall performance and functional failures.  Example - to match memory pads with PCB trace and MCU pads at 50Ohm impedance , we provide IBIS models for PCB simulation. You can refer to this application Note



With very short traces and high speed switching you need some resistance so there's not ringing / standing-wave / reflections in the signals.

Below 50 MHz you can probably get away without them but as you go higher you get into transmission line theory

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Resistors in high speed signal lines help reducing the EMV, because they add an ohmic resistance to the impedance of the trace which helps limiting the current. "high" currents (a few milli-Ampere in this context) create electromagnetic fields around the trace which can radiate off and/or create so called "ringing" - as @Tesla DeLorean already mentioned. "Ringing" means that the signal starts to overshoot and oscillate with a high frequency each time the signal levels changes, driven by the impedance of the trace. It's the same principle as the BackEMF when turning of an inductor, but on a much smaller scale.


In General you don't want to have Ringing or Back-EMF on your PCB, as this could cause many different other issues, besides the unstable or unreliable communication you might experience. You could also run into issues where components get damaged (ok, that's rare) or other components may not work properly.

For my taste everthing above 2 MHz deserves a damping resistor in the signal line. Yes, I am aware of that others may see this as complete overkill, but I made good experiences with that. If you ask the RF guys, 2 MHz is a signal frequency you can even see without a scope and count the oscillations, but I consider 5 MHz SPI or 12 MHz USB as an "entry level high speed signal" where I start to take EMV considerations into account.

It is possible to leave them out, especially at lower SPI speeds, but it does not hurt to have them. I use e.g. 47 Ohm for 5 MHz, 22 Ohm for 10 Mhz and 10 Ohm for 20 Mhz SPI Clocks. That rule of thumb served me pretty well over the years. I never had issues with the SPI signal integrity and it helped to pass the EMV-Tests.

I don't use serial (damping) resistors for UART, CAN and I2C traces below 1 MHz on the PCB.


Thank you @STOne-32 @Tesla DeLorean @CBerg @David Littell For your input, I would just want to clear that Series resistance will be put on the receiver end Right?


nope, you put them on the driver side. For SPI

SCK and MOSI close to the Master

MISO close to the Slave

you want to limit the current that flows into the trace and charges the parasitic capacitance and inductance of the trace. If you take into your consideration that an infinitive steep voltage edge has the frequency "infinity" and a capacitor has the impedance of zero ohms at the frequency "infinity" you can shove unlimited current into the trace at that moment the signal flips. This current had to be provided by the driver circutry if there is no ohmic resistance that limits the "inrush current".


Practically of course there will never be an infinite current, because the driver circuitry itself has some ohmic resistance, too (e.g. the R_ds_ON of the MOSFETs) and the Universe, or at least your MCU would burn when an infinite high current is flowing 😉 But eventually you'll get a relatively high dU/dt (Voltage Gradient) which leads to "electromagnetic radiation" (EMV). With the series reistor on the driver side you flatten that dU/dt.

As the Reveiver input has a relatively high input impedance (Resistance) and the series resistor is low, you have a simple voltage divider. Eventually you drop the signal level by 1%, but that does not matter for the functionality.