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im reading a arm based mcu board schematic and there is 100R resistor (R30) at external 25MHz oscillator source. What is the main reason of using serial resistor at clock line ?

schematic

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  • \$\begingroup\$ electronics.stackexchange.com/a/7737/80284, this resistor will be important if traces are long, or there's significant mismatch in impedance between the endpoints. \$\endgroup\$
    – Anonymous
    Mar 26 '18 at 11:12
  • \$\begingroup\$ Or just for EMI reduction by reducing slope steepness. The schmitt trigger input of the CPU will still produce a clean squarewave as clock. \$\endgroup\$
    – Jogitech
    Mar 26 '18 at 11:16
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    \$\begingroup\$ Reducing reflections caused by a low-impedance source driving a square wave into a high-impedance load. \$\endgroup\$
    – TonyM
    Mar 26 '18 at 11:29
  • \$\begingroup\$ So how can i determine the right value for my design? 100R is typically for it ? \$\endgroup\$ Mar 26 '18 at 11:39