Stm32 mbed tutorial (IV) --pwm

 

Catalog

One .Introduction( Introduce )

Two .Principle( principle )

3、 ... and .How to use in mbed（ Apply to mbed On ） 

Four .Summary（ summary ）

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One .Introduction( Introduce )

PWM Full name Pulse Width Modulation（ Pulse width modulation ）, seeing the name of a thing one thinks of its function , It is to control and interact with peripheral devices by adjusting the pulse width .PWM It is widely used , for example ： The passive buzzer plays a song , Adjust the brightness of the computer screen , Adjust the fan speed , Steering gear rotation, etc ...... PWM The content of the underlying principle of is very detailed and miscellaneous , But the focus of this article is how to mbed Used in program pwm And realize some functions .

Two .Principle( principle )

Here we introduce the knowledge we have learned in junior high school , In a circuit we have to adjust LED The brightness of （ The essence is to give to LED The magnitude of the voltage ）, The relatively simple method is to add a Slide rheostat , Divide the voltage by adjusting the sliding rheostat , So as to adjust the distribution LED The voltage of .

  that PWM How is it right LED For adjustment ？ Let's look at the circuit diagram ：

From the circuit we can find , We have eliminated the use of sliding rheostat ,  But there is a switch in the circuit . Everyone knows the function of the switch , On / off of control circuit , So let's assume a period of four seconds , In the first two seconds, the circuit is open , The circuit is disconnected in the next two seconds , So the first two seconds of this circuit led It's bright , Two seconds later led It's dark , So if we count this 4s The average score of led That's the normal distribution led Half the voltage .（ Of course , The unit cycle time we set here is relatively long , Not enough to say led Brightness adjustment of ） So if we reduce the change period of this voltage , Suppose a period is 4us, front 2us It's bright , after 2us It's dark （ At this time, the human eye has been unable to observe the light and dark changes in a period of time ） So at this time we are 4s Internal observation phenomenon , Will find led It is always on ,（ this 4s Inside LED The voltage on the can be considered as half of the normal voltage ） But the brightness is half the normal brightness .

 

  The Yellow curve in the above figure is the pulse diagram , High level represents high level , Low level means low level . In a cycle , The duration of high level is what we call Pulse width（ Pulse width ）, The proportion of high level in the cycle becomes Duty cycle( Duty cycle ).

 

3、 ... and .How to use in mbed（ Apply to mbed On ） 

As a whole ,PWM It is very simple to use . According to the above principle , We can know that to determine an attribute pwm We only need two necessary factors ：①period（ cycle ）②duty cycle（ Duty cycle ）. Next, let's see how to use mbed Library language .

First we need to define a PwmOut The pin of is used as mbed On pwm Output port of .  that mbed What can be done PwmOut What about the output ？

  Let's look at mbed You can see from the structure diagram of , If there is purple in the pin attribute PWM Can be carried out pwm Output .

Next, we will interpret the case code given by the official ：

/*
 * Copyright (c) 2014-2020 Arm Limited and affiliates.
 * SPDX-License-Identifier: Apache-2.0
 */

#include "mbed.h"

// Adjust pin name to your board specification.
// You can use LED1/LED2/LED3/LED4 if any is connected to PWM capable pin,
// or use any PWM capable pin, and see generated signal on logical analyzer.
PwmOut led(LED1);

int main()
{
    // specify period first
    led.period(4.0f);      // 4 second period
    led.write(0.50f);      // 50% duty cycle, relative to period
    //led = 0.5f;          // shorthand for led.write()
    //led.pulsewidth(2);   // alternative to led.write, set duty cycle time in seconds
    while (1);
}

Here is the designation mbed Upper LED1 The corresponding pin PWM Output （ And named it led）, Next in the main function , First of all led.period（4.0f）; Here is the definition led The period of is 4s（ Floating point numbers ）, Next led.write(0.5f); Definition led The duty cycle of is 0.50f, Indicates half of the high level , Half low level . If we find LED1 Two seconds on and two seconds off . Empathy ： If we want to light up for a second , Dark three seconds , Just adjust the duty cycle to 0.25. Well, if I ask Jean LED1 Always on , And the brightness is half of the original brightness , This time changes period that will do .

Four .Summary（ summary ）

PWM The basic use of is not difficult , There are two main components , Cycle and duty cycle . Through the preliminary introduction of this article , If you are interested, you can use it yourself mbed Do something interesting , Such as the use of pwm Give Way Buzzer Play a song you like , utilize pwm Give Way The steering gear turn , Even adjust the speed of the car .