Guidance on Resistor Value Selection for Different Protocols

[KarthiAru]

Hello everyone!

I've been examining a schematic for a working product and have some questions about the resistor values used for different communication lines and components. Specifically, the schematic uses 220 ohms for UART, 120 ohms for I2C, 22 ohms for switch inputs, and 68 ohms for LED outputs. These components are connected to an STM32 MCU via a mezzanine connector, with the lines being approximately 100mm long.

Are there standard guidelines or rules of thumb for selecting resistor values based on different protocols, or are these values typically determined through experimentation? I'm hoping to streamline my Bill of Materials and reduce the need for extensive testing. Any advice or insights would be greatly appreciated!

[moffy]

The series resistance is for surge/static protection, but the values are somewhat arbitrary in choice, long traces generally require lower values than short ones because of capacitance but that is very layout dependent. Higher speed busses require lower values because of faster transmission speeds/edges. The final choice is a compromise of speed, capacitance and fault voltage tolerance.