How to detect -5 volts?

[rea5245]

Hi,

I have a connector that can plug into two slightly different devices. I want to automatically detect which device it's plugged into. With one of the devices, there is -5V (yes, that's minus 5 volts) coming in on a pin. With the other device, that pin is not connected to anything. I can't connect the pin directly to a microcontroller, because -5V would damage it. What can I do to get a 0 or 1 signal to the MCU based on whether the -5V is there or not?

Thank you,
   Bob

[PartialDischarge]

4.7V zener to the pin, then a pull up 100k resistor to the mcu. If not connected to anything the mcu will see 5V, if -5V is present at the pin, the mcu will see 0

[rea5245]

Will the MCU's built-in weak pull-up suffice, or do I need the 100K resistor?

[schmitt trigger]

The weak internal pullup may work. Having said this, common resistors are ultra-cheap. And if you are using SMT devices, ultra-small too.

My personal opinion is that for this instance I would use an external pullup. But other posters may think otherwise.

[PartialDischarge]

The internal may be enough, but anyway I would also add a series resistor with the zener to limit the current in the mcu, just in case your -5V turn out to be -7 or -10V someday

[coppercone2]

its good form anyway because its a external device and all external IO should be protected if possible by design, it will reduce the ESD some what.

[macboy]

Connect 10k to micro's +5V rail, other end to a microcontroller input pin AND to the device pin via another 10k resistor. When device drives that to -5 V, then the voltage divider makes ~0 V, or logic low to the local micro. Otherwise it floats to +5 V, logic high. A zener (or better, a TVS) to protect the micro's input would be prudent, but its own input protection diodes plus the 10k resistor may be enough.

[spec]

Hi rea5245

Another approach: