CCS/MSP430FR2433: Is it possible to make a GPIO float?

user6202586:

Your design is very close to what will work for you. I'm going to elaborate on what David had advised.

Let's first get our nomenclature correct. GPIO pins are used for producing digital high and digital low voltage signals. Stop using the word float, it's incorrect in this scenario. Furthermore, the digital high voltage signal has a direct and linear relationship with the voltage supplied to the microcontroller (1.8 to 3.6 VDC). When the voltage lowers at the supply, so will the GPIO output voltage lower. If it goes below 1.8, the port pin will no longer be able to produce a signal, nor will the microcontroller operate. So take that into account in your design.

You have a relay, which is normally closed, when there is no voltage applied to its coil, so current is able to flow through the relay switch when closed. When voltage is applied to the coil, the relay switch opens the circuit, and that cuts the flow of current through the relay switch.

Here's the interfacing circuit you need.

For the GPIO pin, a bit-field in the PxDIR register is used for configuring the GPIO pin to produce output signals. A bit-field in the PxOUT register is used for producing digital high and low voltage signals. Meaning, when you set a bit in the proper field, an output voltage is created, and when you clear the bit, you remove the voltage signal.

When the GPIO pin produces a low signal, the NPN transistor is not energized, so current does not flow through the transistor. This scenario removes energy from the coil, so the normally closed relay switch stays closed, which allows x VDC to flow through the relay switch.

When a digitally high voltage signal is applied to the transistor, the NPN transistor allows current to flow from the 12 VDC source to energize the coil, which in turn opens the relay switch. An opened relay switch will open the circuit from x VDC so no current flows through the switch.

Good luck in your endeavor.

-Thomas

Varun:

As an alternative, you could use a silicon controlled rectifier (SCR).

The 100 Ohm resister lowers the output voltage to 1.5 VDC and current to 15 mA. Let's assume the SCR is a SJxx06xx Series by Littlefuse. That SCR can handle 6A at 600V going through it, and it only needs .2 to 15 mA to trigger it. It costs about $2.

Hello Varun,

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