latch up notes


I am trying to read a signal coming from an external circuit thru the use of raspberry pi’s gpio. the signal I am trying to read is a 3.3V signal coming from a voltage divider circuit. this circuit also has an external power source (not coming from Rpi both VCC and GND.) I’m a bit lost to what I am missing. Thanks!

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  • Have you connected the grounds of the Pi and the external device? – joan 1 hour ago
  • @joan yep I connected the ground of the pi and the externad device. I noticed that when I turned on the external device. the Rpi I’m using is somehow turned off. that is I am using a VNC viewer to view that behavior of my code. but as I mentioned when I turn on the device that Rpi is disconnected. – Rick Joseph Amodia 13 mins ago


read signal from external circuit by Rpi GPIO

signal is 3.3V from voltage divider

circuit divider circuit has external power not Rpi

Well, if the divider Vcc is 5V or higher, there is risk that the Rpi will be fried, or life shortened. The cause is usually called the “latch up” effect, when Rpi GPIO is connected by a resistor to 5V or higher, as illustrated below.

A way to get around is to use a digital buffer between Rpi GPIO and the signal to read.

rpi latch up

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