#51 MCU#button

button

There are two commonly used key circuits, one is an independent key and the other is a matrix key.

Independent buttons

The independent buttons are each connected to an independent input line.

Take the circuit in the following figure as an example for analysis.

Independent button schematic diagram:

The basis for judging whether a key is pressed is: the level of the KeyIn (n) IO port.

When K1 is pressed, the high and low level ends of the circuit are connected. At this time, the Keyln1 pin is at a low level.

When K1 is released, the line is disconnected, Keyln1 and +5V are at the same potential, and the Keyln1 pin is at a high level.

Usually, there is a pull-up resistor inside the IO port of the microcontroller.

Take the quasi-bidirectional IO port structure diagram as an example:

The circuit inside the box is the internal part of the microcontroller, and outside the box are the external pull-up resistors and buttons.

Note: When reading external key signals, the microcontroller must first set the pin to 1! ! !

analyse as below:

When the internal output = 1, it is inverted to 0 after the NOT gate, the NPN transistor has no potential difference, and the circuit is not connected. There is no potential difference between VCC (also +5V) and +5V, and the IO port of the microcontroller is at a high level. The key status can be read normally.

When the internal output = 0, it is inverted to 1, the NPN transistor generates a potential difference, and the circuit is connected. The IO port of the microcontroller is affected and becomes a low level, and the key status cannot be read normally.

Matrix Buttons

Based on independent buttons, in order to improve the utilization rate of IO ports, matrix buttons came into being.

As shown in the figure, 8 IO ports realize 16 buttons.

If KeyOut1 outputs a low level, KeyOut1 is equivalent to GND, and the other four keys are equivalent to independent keys. It should be noted that KeyOut2, KeyOut3 and KeyOut4 should all output a high level at this time to avoid affecting the KeyOut1 line.

The circuit diagram is now simplified to this: