I attached a image of ESP32's datasheet where it say to add a series 470 ohm resistor to Tx transmission line. I wonder is it talking about which line, what are the pins?

And why that specific value?

here Is the link to the ESP32 datasheet https://www.espressif.com/sites/default/files/documentation/ESP32_FAQs__EN.pdf

enter image description here

  • \$\begingroup\$ The reference appears to be to the serial UART transmit line, as some combination of slew rate limiting and/or source termination to suppress line reflections. First thing you would have to figure out is if the serial line would be connected in operation, often it isn't but only in debug and test, in which case you primarily need to figure out if it will cause problems then. \$\endgroup\$ Commented Oct 19, 2018 at 16:48
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    \$\begingroup\$ Welcome to EE.SE, @Mano. It's a good idea to crop your photos to show just the relevant section. \$\endgroup\$
    – Transistor
    Commented Oct 19, 2018 at 17:10

1 Answer 1


If you know anything about impedance dividers and the transmission line inductance, capacitance and thus Zo, this is a low Q LPF, easily simulated. For say 220 ohm UTP. Approx. 1~2uH/m & 35pF?? /m. Depending on cable.

DO you know Zc(f)=1/(2pif*C) and when series R= Zc(f) it is at half power and when R=9Zc(f) it is Zc/(Zc+9R)= 1/10th the input V or 20dB down or -20dB.

  • \$\begingroup\$ hi yes i get the point thanks. If I am correct is because digital signal has high order harmonics. the resistors is a simple way to attenuate those high frequency emissions right..?a \$\endgroup\$
    – ManojKumar
    Commented Oct 22, 2018 at 15:29
  • \$\begingroup\$ All square or step functions have harmonics 10~90%[s] Tr=0.35/f [Hz-3dB] with harmonic amplitudes 1/n. Yes cable ,Zo in shunt is mainly 1/2pifC at low f, and R=Rx load (kohm] affect rise time like T=RC with 40~50 pF/m depending twists per m. \$\endgroup\$ Commented Oct 22, 2018 at 16:04

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