I have recently took MCU board and testing continuity across button.

When board is not powered i.e; not executing code, continuity on button is as expected (i.e; diagonally opposite pins are not showing connected). However, while MCU board is executing code and I try to check connectivity across switch pins it shows shorting.enter image description here

Am I doing something wrong? Is there any rule that continuity should be checked only when Board is powered off? Does it means, that board has some fault?

For the reference, I have added snapshot of swich I am using.

  • 2
    \$\begingroup\$ Yes, continuity test should only be performed with power off. Continuity test actually provides a small amount of power from the DMM (e.g. source current and measure voltage to infer resistance), this causes continuity test to interfere with circuit operation. Furthermore continuity test should not be used with microcontrollers or other silicon devices, continuity test may violate abs max ratings and damage the part. If you're lucky it will simply not work but do no permanent damage. \$\endgroup\$ – MarkU Feb 22 '18 at 4:49

The continuity test function on a multimeter is essentially a resistance measurement - the continuity buzzer will sound if the measured resistance is less than some fairly low value - under 40 Ohms for one of my meters.

When measuring resistance, a multimeter will apply some voltage between its leads, and measure the resulting current to determine the resistance (or apply current and measure voltage). If something else is also causing current to flow between the test leads, that current will affect the measured resistance, as the meter can't distinguish between the current it is providing and other current in the circuit.

Therefore, you must never do a resistance or continuity test on a powered circuit.

Also it is important to note when measuring components in-circuit, that the meter will report the effective resistance between the test leads, which may include other components in parallel with the component you are intending to measure (and touching both probes with your fingers can also upset the reading, due to the resistance of your body)


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.