Why is it necessary to add resistors in series with switch?

Thread Starter

Guru_007

Joined Mar 24, 2021
7
Hi,
I'm unable to understand why resistors are connected in series with switch (R36 & R37). Kindly share your thoughts. I'm attaching a reference schematic below.

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Thanks
 

Papabravo

Joined Feb 24, 2006
21,008
You never want I/O lines on a processor to "float" at an undefined voltage level. The 56KΩ resistor to 3.3 V is a "weak" pullup. The 2.2KΩ is a "strong" pulldown to GND. The terms "weak" and "strong" in this context are relative.
 

Thread Starter

Guru_007

Joined Mar 24, 2021
7
You never want I/O lines on a processor to "float" at an undefined voltage level. The 56KΩ resistor to 3.3 V is a "weak" pullup. The 2.2KΩ is a "strong" pulldown to GND. The terms "weak" and "strong" in this context are relative.
I agree 56k is required for known state, but when switch is connected to GND why do we need a series resistor?
 

Jon Chandler

Joined Jun 12, 2008
988
but when switch is connected to GND why do we need a series resistor?
If the GPIO pin is inadvertently set to be a high output, the high output would be shorted to ground when the switch is pressed.

I must admit, it's not a fault condition I've ever worried about. The important point is that the voltage divider formed by the two resistors when the switch is pressed allows the I/O line to be less than the maximum voltage that's considered a low level.
 
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