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This article mainly shows you the "STM32 touch button example analysis", the content is easy to understand, clear, hope to help you solve doubts, the following let Xiaobian lead you to study and learn "STM32 touch button example analysis" this article.

01. Touch button principle

Touch uses the principle of RC charge and discharge:

RC circuit is a circuit composed of resistance R and capacitor C. it is a commonly used circuit in pulse generation and shaping circuit.

Charging process:

The power supply charges the capacitor through the resistor. At first, the voltage at both ends of the capacitor is 0, so the voltage is on the resistor, so the current is large and the charging speed is fast. With the increase of the voltage at both ends of the capacitor, the voltage at both ends of the resistor decreases, the current decreases, and the charging speed is small. The speed of charging is related to the size of resistance and capacitance. The greater the resistance R, the slower the charge, and the larger the capacitance C, the slower the charge. The constant t (tao) = RC to measure the charging speed.

Discharge process:

The capacitor C discharges through the resistance R, because the initial voltage of the capacitor is E, the discharge current is I=E/R, the current is very large, so the discharge speed is very fast. With the continuous discharge of the capacitor, the voltage of the capacitor decreases. The current also decreases quickly. The discharge speed of capacitor is related to RC. The larger the resistance value of R is, the slower the discharge speed is. The larger the capacitance is, the slower the discharge speed is.

RC circuit charge and discharge formula:

Vt = V0 + (V1-V0) * [1-exp (- t/RC)]

V0 is the initial voltage on the capacitor

V1 is the voltage value to which the capacitor can eventually be charged or put.

Vt is the voltage on the capacitor at t time.

If V0 is 0, that is, charging starts at 0V. Then the formula is simplified to:

Vt= V1 * [1-exp (- t/RC)]

Conclusion: under the same conditions, the capacitance value C is proportional to the time value t. The larger the capacitance is, the longer it takes for the charge to reach a critical value.

02. Circuit design

The circuit design is actually a pull-up resistor.

PCB design, a direct circle, and the underlying ground copper clad to form a stray capacitor.

Principle of capacitive touch button

R: external capacitor charge-discharge resistance.

Stray capacitance between Cs:TPAD and PCB.

Cx: the capacitance between the finger and TPAD when the finger is pressed.

Switch: capacitor discharge switch, replaced by STM32IO port.

03. Code design

Detect the process of capacitive touch keypad

① TPAD pin is set to push-pull output, output 0, realize capacitance discharge to 0.

The ② TPAD pin is set to floating input (the state after IO reset) and the capacitor begins to charge.

③ also turns on the input capture of the TPAD pin and starts the capture.

④ waits for charging to complete (charging to the end of Vx, rising edge detected).

⑤ calculates the charging time.

Touch button initialization

Uint8_t Touchpad_Init (void) {uint16_t buf [10]; uint16_t temp; uint8_t j Magi; TIM_ICInitTypeDef TIM_ICInitStructure; / * TIM12Configuration * / TIM_Config (); TIM_ICInitStructure.TIM_Channel = TIM_Channel_1; TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising; TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI; TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1; TIM_ICInitStructure.TIM_ICFilter = 0x0 TIM_ICInit (TIM12, & TIM_ICInitStructure); / * Enablethe CC1 Interrupt Request * / TIM_ITConfig (TIM12,TIM_IT_CC1 | TIM_IT_Update, ENABLE); / * TIM enablecounter * / TIM_Cmd (TIM12, ENABLE); for

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