1 Introduction
Compared with mechanical buttons and resistive touch buttons, capacitive touch buttons are not only durable, low-cost, simple and easy to install, waterproof and antifouling, but also provide functions such as rollers and sliding bars. However, capacitive touch buttons also have many problems. Because there is no mechanical structure, all detections are small changes in power, so they are much more sensitive to various interferences. ST has introduced a capacitive touch sensing solution based on the STM8 series 8-bit general-purpose microcontroller platform for home appliance applications, especially induction cooker applications. It does not need to add a dedicated touch chip, and only a simple peripheral circuit can realize the capacitive touch sensing function. Convenient for secondary development of customers.
2 Introduction
ST's capacitive touch key solution detects the change in capacitance caused by human touch through the charge / discharge time of a resistance-capacitance network formed by a resistor and the capacitance CX of the sensing electrode. As shown in FIG. 1, when the human hand presses, it is equivalent to a capacitor CT connected in parallel to the sensing electrode, which increases the capacitance on the sensing electrode, and the time for the sensing electrode to charge and discharge will increase, thereby detecting the state of the button. The sensing electrode can be directly drawn on the PCB board as the application style of buttons, rollers or sliding bars, or it can be made into a spring member inserted on the PCB board, even if it is separated by an insulating layer (glass, resin) Has an impact.
Figure 1 The working principle of STM8S capacitive touch buttons
Induction cookers use the heating principle of magnetic field induction current to heat food. When heating, a strong magnetic field is generated by the coil below the panel, and when the magnetic field lines pass through the bottom of the pot made of the magnetic conductor, the pot cuts the alternating magnetic lines of force and generates eddy currents at the bottom of the pot to rapidly heat the bottom of the pot to achieve the purpose of heating food. In this solution, the 44pin STM8S105S4 is used as the main control chip of the button display board, which controls the scanning of 13 buttons, the display of 24 LEDs and a 4-digit digital tube, the communication between I2C and the main board, and a SWIM interface is convenient Engineers use it for debugging (see Figure 2).
STM8S105S4 uses ST advanced STM8 core, with a 3-stage pipeline Harvard structure, 3.0 ~ 5.5V operating voltage, internal 16MHz RC can provide MCU 16MHz operating frequency, provides low power consumption mode and peripheral clock shutdown function, a total of 34 I / O is available. STM8S105S4 has 2KB of RAM and 16KB of FLASH, as well as 1KB of EEPROM data memory with up to 300,000 erase and write cycles.
Figure 2 The principle of induction cooker key board
USB charger, USB Adaptor, Charging USB Ports, USB quick charger
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