1

Almost always in electronic designs resistors are used in series in the SPI/UART/USB connections to avoid overshoot, and they are encountered in values ​​between 10R and 350R. With what criterion should these values ​​be chosen? The high values ​​should have better performance, but having never seen high resistance values, ​​I suppose you should never exceed the kiloohm range. Could it be due to the fact that the resistors together with the parasitic capacitance of the copper trace and the pins create a "weak" low pass filter that interferes at very high frequencies (personal guess)?

Context:

  • circuit PCB
  • SPI, clock 40 MHz, maximum track length 50/100 mm
  • UART, maximum track length 50/100 mm
  • USB, maximum track length 30/50 mm
Null
  • 7,448
  • 17
  • 36
  • 48
boromyr
  • 15
  • 1
  • 5

1 Answers1

1

There can be no one single answer, except a very generic one, since you categorized three completely different kinds of buses together.

The resistors must be low enough in value to not cause other aspects of the signal to go out of spec. It includes DC high and low voltage levels, signal bandwidth, rise and fall time, and eye pattern. And they are specific to which impedance the PCB tracks are designed for, and what is the output strength or impedance of the IO pin drivers.

Yes too large value and signal rises too slowly due to stray capacitances too, but that's only the first order RC effect.

Signals also travel on PCB tracks and reflect on discontinuities such as end of wire, so the added resistor couples as a crude source impedance matching too.

USB is a very well defined electrical interface so there is not a lot of room for custom values, usually you put what your chip manufacturer suggests.

The other buses have more room for adjusting the values based on speed and pull-up resistances.

Justme
  • 127,425
  • 3
  • 97
  • 261
  • Thanks for your kind explanation. I mentioned 3 buses because as you said the specifications can change, and because they are the most common ones. So what values ​​are you referring to for USB? – boromyr Feb 23 '22 at 18:38
  • None, because there are no general values. Always use values recommended by manufacturer for the specific chip - which includes the case of no resistors recommended at all. – Justme Feb 23 '22 at 19:33
  • Okay, thanks anyway. – boromyr Feb 24 '22 at 09:42