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I am trying to build my own ESP32 based PCB. I tried to copy the USB-to-serial circuit from the official DevKit. There I see two 0Ω resistors. Why did they add them? What do they do?

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USB to Serial datasheet (cp2102n): https://www.silabs.com/documents/public/data-sheets/cp2102n-datasheet.pdf

Full ESP32 DevKit datasheet: https://dl.espressif.com/dl/schematics/esp32_devkitc_v4-sch.pdf

My PCB schematics and layout so far: https://github.com/crgarcia12/electronics-homeassistant-lightscontroll/tree/main/PCB/schematics-v16-espinside

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3 Answers 3

Reset to default
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They allow users to remove resistors to disconnect the ESP from CP2102 and connect them for other purposes.

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  • \$\begingroup\$ just out of curiosity.. Do you know any example of "other purposes"? \$\endgroup\$
    – Carlos Garcia
    Commented Dec 26, 2021 at 20:22
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    \$\begingroup\$ Since it is a dev kit, you might buy it and want to connect it to some other device which uses the serial port. If there is no easy way to detach the CP2102 from the serial port, it is much harder and more permanent solution to cut the PCB wiring with a knife or desolder the CP2102, and then it would be quite hard to undo that change to reprogram the ESP. \$\endgroup\$
    – Justme
    Commented Dec 26, 2021 at 20:35
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    \$\begingroup\$ Non-EE guy here... What's the difference between a 0 ohm resistor and just a jumper wire? \$\endgroup\$
    – 640KB
    Commented Dec 27, 2021 at 14:50
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    \$\begingroup\$ @640KB The 0Ω resistor is compatible with automatic surface-mount fabrication. \$\endgroup\$
    – John Doty
    Commented Dec 27, 2021 at 14:58
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    \$\begingroup\$ @johndoty follow up question - how does a 0 ohm resistor have a 5% tolerance? I understand even a jumper wire will have some resistance but 5% of 0 seems a bit difficult to measure! \$\endgroup\$
    – 640KB
    Commented Dec 27, 2021 at 15:15
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Developers might changed schematic without changing PCB. Usual practice.

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Some microcontrollers (such as the ATmega 16U2) require a 22 Ohm series resistor on the D+ and D-. Perhaps the designer wasn't sure if a series resistor would be required so they specified a 0 Ohm resistor. It is a lot easier to desolder that resistor and replace it with the correct value than to add one after the fact if something does not work as expected.

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