Dealing with unused pins of microcontrollers

Usually we don't use up all the pins of the microcontroller, and sometimes we also have high and low configurations on the same product, so how to treat some unused pins becomes a problem. First, let's take a look at the structure of the microcontroller pins.

We can find that the IO ports of general microcontrollers can be configured.

CMOS device pins are composed of N-channel and P-channel field effect transistors. Usually, only one tube is turned on at a time, but there is a very short transition period when both tubes are partially turned on, when one tube is turned off and the other is turned on. An unterminated input port may oscillate or float at an intermediate level, causing all field effect tube devices to be in a slightly turned-on state, resulting in tube losses, increased noise, and consumption of additional power supply current.

(Input Only) Input-only pin

1. Use pull-up or pull-down resistors to pull each unused pin to VSS or VDD. This is the recommended method.

2. Bundle together and use a single resistor to pull up/down to VDD or ground, saving component count and cost, but this reduces flexibility.

(Input/Output) Input/Output Pin

1. Pull down each pin with a resistor to ground (VSS).

2. Pull several pins down to ground (VSS) with the same resistor.

3. For unconnected pins, the software program configures the IO ports as output ports and sets them to output low.

4. Connect a resistor to Vss and configure the IO port to output low.

5. Connect directly to ground, the software program configures the port as a high-impedance input port and sets the register to output low state.

Note that this will cause problems due to double faults (a pin changing from input to output and an output changing from a low to high state).

The risks are:

Finally, I think it is safer to connect each resistor to ground.