First, you must determine what the load requirements are (i.e. the current that will be required). You must refer to the data sheet for your particular µC to determine what the output current for your I/O is. It can only deliver so much and does have a limit. Your I/O can drive other TTL/CMOS devices. Things of low current a so forth. For larger loads, such as a motor, will require extra drive capability. For instance, you would have to use MOSFET transistors capable of handling the current requirements of your load. The digital I/O would control the on/off function of the transistor (so a low current device controls a high(er) current device). Your µC uses transistors to drive the lines, however they are not as "big" and cannot handle as much current as a power transistor. Again, refer to the data sheet for your µC.

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In a perfect world, we could maintain the voltage and the current would take care of itself. However, this isn't a perfect world. We can only build things that do this "within limits". For example a microcontroller pin will provide 5V at any current...as long as the current is 20mA or less. It's the same thing as how a 12V power supply rated for 20A is able to provide 12V at any current less than 20A.

So for a brushed motors (which commutates themselves so you just give them straight power and they run) you can use a power N-channel MOSFET transistor where the source-drain are in series with the gate being controlled by the MCU pin. THe NMOS is placed nearest to ground so that it connects and disconnects the motor from ground- this is the simplest arrangement because the voltage that controls the MOSFET is between it's gate and source pin, and since the MCU pin voltage is relative to ground then if you connect source to ground then the gate voltage and MCU pin voltage have the same reference. THis only provides unidirectional control though.

FOr bi-directional control you need more complicated transistor arrangements like an H-bridge and support circuitry to switch the power MOSFETs. THe support circuitry mainly does two things:
-In more complicated MOSFET arrangements, it is almost guaranteed that some MOSFETs will not have their source pin connected to ground. Because now the reference point for the voltage controlling the transistor isn't the same as the reference voltage that the MCU pin is using anymore, you need some circuitry to take the voltage between MCU pin and ground and "float it up" to become the voltage difference between the MOSFET's gate and source pin.

- to make the MOSFET turn on faster and spend less time in between on and off where it wastes the most energy and heats up the most. THis is important for high frequency switching for things like speed control where the MOSFET spends a lot of time moving between on/off and does so frequently.

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