Interface application between capacitive touch sensing key chip and single chip microcomputer

1、 Overview:

General key switches use two metal contacts to make electrical conduction under the action of external force to form an electrical circuit to realize current transmission. This kind of key switch is characterized by simple structure and low price, so it is widely used in electrical equipment. However, because this kind of key switch has mechanical contacts, it will have the problems of oxidation and poor contact after a long time, The service life is very limited

Therefore, some electrical equipment also use metal induction keys, but these keys still have exposed metal parts, which still need direct contact with human body, and the potential safety hazards and application limitations can not be completely eliminated

Capacitive touch sensitive key is a new type of key developed rapidly in recent years. It can penetrate several millimeters of insulating materials, detect the moving charge induced by the human body, judge the finger action of the human body, identify the "on" and "off" of the key, realize the key function, completely solve the oxidation and poor contact problems of ordinary mechanical contact keys, and greatly improve the service life, The key panel can be a flat surface without gaps, which is easy to clean or even wash. This is very suitable for the use of some kitchen appliances (such as induction cooker and microwave oven)

Here we introduce the interface application between the tch683 capacitive touch sensing key chip and the single chip microcomputer. At present, there are many chips with the same function in the market, such as mt8810, etc. the circuit block diagram is as follows:

As can be seen from the above figure, there are almost no external components on the periphery of the touch chip. Only one resonant capacitor needs to be connected. The key IC output is connected to the MCU through four buses. The output of the tch683 is BCD code, which is easy to connect with the MCU bus interface. The working voltage range of the tch683 is 2.2-5.5v, the reflection speed is about 0.2S, the maximum time limit of the key is about 15s, and the chip has a certain EMC (electromagnetic compatibility) characteristics, meeting general electrical requirements, no problem

The following figure shows the appearance of a control motherboard PCB using capacitive touch sensing keys designed and produced by our company

For the metal sensing part of the key, we use the spiral spring style. When fixed on the panel, it is close to the key panel. The key value can be obtained only when the finger is close to the key. The single chip microcomputer can read the key state, so as to realize the control function

Due to such a sensing structure, there will also be some design inconvenience. For example, the keys cannot be too crowded, and there must be a certain space, which generally needs to be 25 mm apart, otherwise there will be interference or induction mixing. This is a fly in the ointment. It is only suitable for occasions where the panel does not have too many space restrictions

The figure below shows the component surface appearance of the key board