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Nick_Ashley

Maximum Safe Current for UC32 Digital Pins

Question

Dear Digilent Support Staff,

I'm a graduate student at the University of Arizona, and for my Master's Project, I'm designing an irrigation process control system.

 
I'm using a Digilent UC32 microcontroller to read whether or not a switch from a relay contact from a flow meter is either open or closed. I will be using a 5 V power supply, and have read that the digital input pins on the Digilent UC32 can handle 5V at logic levels. I have a few questions:
 
What is the maximum current that is safe for the UC32's digital pins to handle?
Do the digital pins have a built-in resistor that will limit the current draw, or do I need to use a voltage divider for this?
What is the minimum current that the UC32's digital pins can take accurate HIGH/LOW input readings at, from a 5V power source?

Best Regards,
Nick

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Hi @Nick_Ashley,

Looking at the PIC32 datasheet on section 29.1 there is not a minimum current but there is a min and max input voltage for detecting high and low logic. In the uc32 reference manual on page 8 under section 3.3 5 volt compatibility it states that there are current limiting resistors. And that the digital I/O is 5 volt tolerant.

thank you,

Jon

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5 minutes ago, jpeyron said:

Hi @Nick_Ashley,

Looking at the PIC32 datasheet on section 29.1 there is not a minimum current but there is a min and max input voltage for detecting high and low logic. In the uc32 reference manual on page 8 under section 3.3 5 volt compatibility it states that there are current limiting resistors. And that the digital I/O is 5 volt tolerant.

thank you,

Jon

Hi Jon,

So suppose I connect my switch to the positive terminal of a 5V, 120 W power supply (from which many other components will be receiving 5V power) and connect the other side of the switch to a voltage follower composed of a 1kOhm and a 1MOhm resistor, and then the other end of the 1MOhm resistor to ground.I
If I then use the UC32 to measure the voltage at a point between the two resistors using a digital I/O pin, will the device read a 4.995 V (HIGH) when the switch is closed? Or do I only need one resistor and take the digital reading upstream from the resistor?

Best Regards,
Nick

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Hi @Nick_Ashley,

Make sure you are using the digital I/O and I do not believe you will have an issue if you are measuring between the resistors or after the Voltage follower. I believe it should show the 4.995 V (HIGH) when the switch is closed.

cheers,

thank you,

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On 3/26/2018 at 3:27 PM, jpeyron said:

Hi @Nick_Ashley,

Make sure you are using the digital I/O and I do not believe you will have an issue if you are measuring between the resistors or after the Voltage follower. I believe it should show the 4.995 V (HIGH) when the switch is closed.

cheers,

thank you,

Hi Jon,

Thank you very much for your help with my questions! I believe that we will be able to use the UC32 to serve as a pulse counter after all.

Thank You,
Nick

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