Common methods and precautions for microcontroller output PWM

The application of PWM can be said to be very wide, controlling motor speed, light brightness, communication modulation and many other fields.

My friends have asked me a lot about PWM, and I have been using PWM recently. Here I will share some information about PWM.

What is PWM?

PWM: Pulse Width Modulation, pulse width modulation.

There are many explanations on the Internet. Through the picture below, you can intuitively understand PWM, which is actually a pulse signal composed of high and low levels.

By changing the frequency (pulse period) and duty cycle, it can be applied in many situations.

Common PWM output methods

From the above description, PWM is an IO port that outputs high and low levels at different time periods.

1. In the novice (rookie) level while loop, blocking delay controls the high and low output of the IO port:

Blocking delay can be: software simulation delay, timer blocking delay, etc.

2. In the entry (junior) level while loop, non-blocking delay controls the high and low output of the IO port:

Non-blocking delays can be: timer identification detection, RTOS (system) delays, etc.

3. Familiar with (intermediate) level timer interrupt control IO high and low level output:

Timer interrupt configuration——>Start timer——>Respond to interrupt and control IO high and low levels...

4. Proficient (intermediate+) level

Timer PWM hardware control output:

Configure the IO corresponding to PWM and timer PWM output——> Start PWM automatic output...

Comparison:
Among the above PWM output methods, the first three will cause the CPU to interfere with the PWM output, which means it will occupy CPU resources. Especially the first two methods, not only occupy the CPU, but also have a relatively large error.

Using the third interrupt method, if the frequency is relatively high, the CPU consumption will be serious. This situation is suitable for microcontrollers without hardware PWM output.

The fourth type is that the microcontroller has its own hardware PWM output function. It only needs simple configuration to automatically output PWM waveforms without CPU intervention.

Hardware output PWM example

Here we take the familiar STM32F1 as an example: I will briefly share with you the hardware timer output PWM waveform.

PWM timer related macro definitions:

PWM configuration:

PWM output function interface:

Initialize the configuration, call the function interface, and directly output the PWM waveform:

Output PWM waveform:

illustrate:

This example uses the STM32 standard peripheral library. If you want to understand the principles in depth, it is recommended to use the standard peripheral library.

Of course, if you want to use the PWM function quickly without understanding its principles, you can directly use the STM32CubeMX configuration to generate code:

Configuration considerations

If you want to control it more accurately and better meet the needs of the application layer, you need to understand the principles step by step.

Let’s talk about some common questions below.

1. Pin mapping

If the pins you use need to be mapped, you need to configure the corresponding parameters.

For example: STM32F1 uses PB11 (need to check the data sheet):

The corresponding "mapping" code needs to be added:

2. Frequency and duty cycle accuracy

If a 32-bit timer is used, the frequency range is wider and the accuracy can be higher. For example: frequency: 0.01Hz, duty cycle 0.01%, etc.

If it is 16 bits, none of the parameters can exceed 16 bits (65535):

You can configure it according to your own situation, such as PWM (timer) counting clock, frequency division value, etc.

For actual application code, it is recommended to add judgment on each parameter to prevent crossing the boundary (here, for the convenience of understanding, it is written relatively simply).

3. More STM32 have hardware PWM output function, but different series may have slightly different configurations. Simply refer to the official routines and manuals.

Nowadays, most microcontrollers have their own hardware PWM output function. The advantage of the hardware is that it does not require CPU intervention. If not, you can try the timer interrupt method mentioned above.