Generate PWM signals in STM32 microcontrollers – Fusion of Engineering, Control, Coding, Machine Learning, and Science

Figure shown below illustrates a Pulse Width Modulation (PWM) signal.

Figure 1: Illustration of the PWM signal.

The PWM signal is defined by the following variables

$s$ – length of the pulse
$T_{\text{PWM}}$ – period of the PWM signal.
$V_{\text{max}}$ – maximal (voltage) value of the PWM signal.

The frequency of the PWM signal is

(1) $\begin{align*}f_{\text{PWM}}=\frac{1}{T_{\text{PWM}}}\end{align*}$

The duty cycle is defined by

(2) $\begin{align*}D=\frac{s}{T_{\text{PWM}}} \end{align*}$

In the STM32 development environment, we select the frequency of the PWM signal by using the three parameters $f_{\text{TC}}$ , $k_{\text{PS}}$ , and $k_{\text{P}}$ , and the following equation

(3) $\begin{align*}f_{\text{PWM}}=\frac{f_{\text{TC}}}{(k_{\text{PS}}+1)(k_{\text{P}}+1)} \end{align*}$

where

$f_{\text{TC}}$ is the frequency of a timer clock of an STM32 microcontroller.
$k_{\text{PS}}$ is the prescaler value.
$k_{\text{P}}$ is the period value.

The frequency the time clock $f_{\text{TC}}$ of the clock is measured in MHz. Usually, every STM32 microcontroller has a maximal value. The prescaler value $k_{\text{PS}}$ is used to downscale the frequency of the time clock. The period value $k_{\text{P}}$ is the final parameter that determines the frequency of the PWM signal.

In the STM32CubeIDE development environment:

The period $k_{\text{P}}$ is called “Auto Reload Register” which is abbreviated by ARR. That is, $ARR=k_{\text{P}}$ .

In the STM32CubeIDE environment, the duty cycle is determined by

(4) $\begin{align*}D=\frac{CCR}{ARR+1} \end{align*}$

where $CCR$ is the Capture/Compare Register (CCR). We select the value of $CCR$ in the interval $0,1,\ldots, ARR+1$ .

Let us illustrate this with an example. Let us suppose that we want to generate a PWM signal with the frequency of $10,000$ $\text{Hz}$ . Let us suppose that the time clock frequency is $84$ $MHz$ . Then, by selecting the prescaler value of $k_{\text{PS}}=83$ and the period of $ARR=k_{\text{P}}=99$ , we obtain the desired frequency