Is maximum/minimum voltage always compared as difference voltage for a microcontroller?

Question

Let's take the ATmega328P for example. It has a minimum input voltage of -0.5 and maximum input voltage of Vcc + 0.5.

But what if I say Vcc = -5 V and GND = 0 V? Will it break then, because the minimum voltage is lower than -0.5V?

How can I protect the inputs from negative voltages?

This is a way to protect a MCU pin e.g ADC input from high voltage, but it does not protect if I set GND to -10 V. The linear voltage regulator has a diode as protection, but I don't know if that helps.

Update:

Let's say that we have three cases:

Case 1: If A = Positive, B = 0V and C = High, then the zener diode will protect. - OK!

Case 2: If A = 0V, B = Positive and C = 0V, then current is going to flow from B -> C and leaving a voltage drop at A0 depending on how large R1 is. This can hurt my analog input A0 - How can I protect it in that case?

Case 3: If B = Negative and A = Positive, this will hurt my MCU directly. How can I protect it in this case?

Answer 1

How can I protect the inputs from negative voltages?

Using diodes and Zener's is not accurate when protecting inputs for your MCUs. If you want to protect the input then use a device such as the TLV6001 to buffer the input signals. This is the only way to provide professional protection that is viable for both digital and A/D inputs.

A simple circuit such as this will provide accurate translation of the 0-5v signal for any application.

^{simulate this circuit – Schematic created using CircuitLab}

The TLV6001 (or TLV6004) is rated for rail-rail operation and so allows the full range of MCU range for A/D input ...in addition the device is rated to carry 10mA in the input protection diodes, though you need to provide the series resistor. This allows the configuration shown above to withstand voltages of +/-200 V on the input resistor R2 (providing your resistors are rated for this voltage). If you only need protection for say 24-58VDC you can reduce the input resistor to suit your need.

Since the TLV600 is powered by the MCU +5 V supply it cannot produce an output voltage above 5 V or below 0 V, so the input is accurately clamped WITHOUT impacting the A/D range at all, and of course this would work for digital inputs just as well. You must make sure that your MCU solution is always drawing a minimum current greater than your expected protection current (this is only an issue if you put things into a sleep state).

You always need to provide a good ground/0V reference on your interface, it is never a good idea to capture signal relative to a power supply rail.

Answer 2

But what if I say Vcc = -5V and GND = 0V volt? Then it will break?

Yes, it will break.

You will be forward biasing the ESD protection diodes on every IO pin, and sending uncontrolled current through them. This will almost certainly damage the chip within a fraction of a second.

How can I protect the inputs from negative voltage?

You can connect a Schottky diode with its cathode connected to the data line and anode connected to ground (the same node as the ground pins of the uC). Use a resistor in series with the signal to limit current through the Schottky diode.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

Voltage is relative, you are correct there. BUT you can't put things in opposite polarity, because in the context of the microcontroller, you're going outside it's operating parameters according to the datasheet.

But what if I say Vcc = -5V and GND = 0V volt? Then it will break?

Yes, it will break.

Now an interesting thing you can do with this knowledge is to run the microcontroller with Vcc=0V and Gnd=-5V. Note, this doesn't violate the datasheet, and note the polarity! 0V is greater than -5V. Also, if you were to do something like this, you'll need to run your entire circuit at these shifted voltages, or else make a properly designed voltage translator to interface to something that is running at the more common Gnd=0V reference.

EDIT: After seeing more comments on the OP, this answer doesn't really apply to the OP, but leaving it for anyone else who needs to do some non-common circuit design.

Answer 4

This is a solution. I use a PNP transistor. Now when I apply -10V, then I get very low voltage to the MCU pin. If I apply +10V, then I get 5 voltage to the MCU pin because the zener diode is at 5V level.

Or one could use a simple "fools diod" between ground and input. Then all the current will flow from GND to the input.