LM555: Why falling edge signal on reset pin does not make output low?

Xiaoming Wang

Part Number: LM555

Hi,

I feed 2 falling edge signals to pin 2(trigger) and pin 4(reset). I want output high whenever there is a falling edge signal on pin 2 and output low whenever there is a falling edge signal on pin 4.

Pin 6 and 7 are floating.

Here is what I get.

Yellow is trigger pin. Blue is reset pin. Purple is output pin.

Why the falling edge signal on reset pin does not make output low?

Regards,

Xiaoming

over 5 years ago

0 Xiaoming Wang over 5 years ago

Intellectual 490 points

Problem solved.

The voltage on reset pin should go to 0 to trigger reset. Even if there is 1 volt remaining, it is not going to work.

Hope this can help others.

Xiaoming

0 Katlynne Jones over 5 years ago in reply to Xiaoming Wang

TI__Mastermind 41573 points

Hi Xiaoming,

Thank you for posting your solution. Please let us know if you have any other questions.

Per the datasheet, at 25C any value below 0.4V will trigger a reset. This value varies with temperature, so pulling reset all the way to 0V is the safest option to guarantee a reset condition. A value above 1V will release the reset.

Best,

Katlynne Jones

Clock & timing

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LM555: Why falling edge signal on reset pin does not make output low?