Weird behavior of 555 timer discharge pin


I was playing around with 555 timer circuits and I noticed something I was not expecting in the simulation of the discharge pin.

According to all my references, the discharge pin is supposed to provide a low impedance path to GND whenever the 555 output is high and the capacitor needs to discharge; high impedance otherwise.

So if I uncouple the discharge pin from the rest of the circuit (for instance by discharging the capacitor through the output pin) and add a pull-up resistor, I should see it mirroring exactly the output pin.

Instead I see weird spikes:

Is this a bug in CircuitLab's 555 model? Am I using it wrong? What's going on?

Example circuit:

by etatoby
October 16, 2021

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