STM32 PWM Frequency & Resolution Configuration

In this tutorial, we’ll discuss The STM32 PWM Frequency & Resolution Configuration, and how to calculate and configure the STM32 PWM to achieve a specific output frequency and/or resolution. You’ll learn why it’s important to have a high PWM resolution and what it means for your target application. Without further ado, let’s get right into it!

Table of Contents

1. STM32 PWM Frequency
2. STM32 PWM Resolution
3. STM32 PWM Frequency & Resolution Control Example
4. Wrap Up

STM32 PWM Frequency

Pulse Width Modulation (PWM) is a technique for generating a continuous HIGH/LOW alternating digital signal and programmatically controlling its pulse width and frequency. Certain loads like (LEDs, Motors, etc) will respond to the average voltage of the signal which gets higher as the PWM signal’s pulse width is increased. This technique is widely used in embedded systems to control LED brightness, motor speed, and other applications.

The Main 3 Parameters of a PWM Signal are:

1. PWM Frequency
2. PWM Duty Cycle
3. PWM Resolution

In this tutorial, we’ll dig deeper into the PWM frequency and resolution to learn more about controlling each parameter and what is the relationship between the two quantities.

STM32 PWM Frequency

Generally speaking, the PWM signal’s frequency is a measure of how fast the PWM signal keeps alternating between HIGH and LOW. The frequency is measured in Hz, and it’s the inverse of the full period time interval. Here is how it looks graphically and its mathematical formula.

in STM32 microcontrollers, the PWM signal’s frequency F_PWM can be controlled (configured) by adjusting one or multiple of the following parameters:

• Timer’s Clock Source Frequency (F_CLK)
• Timer’s Prescaler (PSC)
• The Auto-Reload Register (ARR)

The mathematical formula below governs the relationship between the output PWM signal’s frequency (F_PWM) and the other three parameters.

For instance, if we have an STM32 microcontroller with an F_CLK of 72MHz. And we’d like to generate a 1kHz PWM output signal. We’ll have to calculate the values of ARR & PSC to achieve the desired F_PWM output.

There are too many solutions for such a problem, of which the following sets:

• ARR = 35999, PSC = 1
• ARR = 23999, PSC = 2
• ARR = 17999, PSC = 3
• ….
• ….
• ARR = 1799, PSC = 39
• and so on …

Plug in each set of (ARR & PSC) values into the F_PWM general equation shown above, and you’ll always get a 1kHz output frequency!

You can just use any combination of the (ARR & PSC) listed above and you’ll always end up having a 1kHz PWM output signal. However, among all possible solutions, only the first one is considered the best since it gives us the maximum PWM Resolution, which is always a desirable target to achieve as we’ll discuss further in the following section hereafter.

STM32 PWM Resolution

The PWM resolution is expressed in (bits). It’s the number of bits that are used to represent the duty cycle value. It can be 8bits, 10, 12, 16, or anything in between. The PWM resolution can be defined as the number of discrete duty cycle levels between 0% and 100%. The higher the PWM resolution, the higher the number of discrete levels over the entire range of the PWM’s duty cycle.

A PWM resolution of only 3 bits means there are only 8 discrete levels for the duty cycle over the entire range (from 0% up to 100%). On the other hand, a PWM with a resolution of 8 bits will have 256 discrete levels for the duty cycle over the entire range (from 0% up to 100%). You can use the interactive tool below to test this yourself.

Change the duty slider to control the LED brightness while using the default PWM resolution of 3 bits. Then, change the resolution up to 8 bits or more and try changing the duty cycle again and see how it becomes much smoother with a lot more degrees of control over the duty cycle (Higher Resolution).

STM32 PWM Resolution

In STM32 microcontrollers and all the other MCUs, the PWM resolution is not a parameter that you can independently control. It's a function of both frequencies (F_CLK and F_PWM). To be precise, here is the mathematical formula for PWM resolution calculation equations:

1. Resolution Calculation Equation

2. Resolution Control Equation

Both equations are essentially the same. However, the first equation is used to evaluate the PWM resolution at a given setting (F_CLK and F_PWM). The second equation is useful to help you set up the PWM in such a way that gives you a desired resolution regardless of the F_PWM of your output signal.

STM32 PWM Resolution Calculation

To practice the PWM resolution calculation steps, let's just recall the 1kHz PWM output example where the F_CLK was 72MHz. The first set of ARR & PSC values that satisfy the F_PWM equation was:

• ARR = 35999, PSC = 1

Therefore, under these configurations, the F_PWM is actually 1kHz as desired. And the PWM resolution is given by:

Resolution_PWM = log₂(ARR+1) = log₂(36000) = 15.1357 [Bits]

In other words, there are 36000 discrete levels of control over the PWM's duty cycle.

Recall my comment about that first set of (ARR & PSC) values being the best among the others. Let's now consider the last set of (ARR & PSC) values that also give us the desired F_PWM of 1kHz.

• ARR = 1799, PSC = 39

Likewise, under those configurations, the F_PWM is also 1kHz as desired. And the PWM resolution is given by:

Resolution_PWM = log₂(ARR+1) = log₂(1800) = 10.8138 [Bits]

As you can see, it's way fewer degrees of control over the PWM's duty cycle. Only 1800 levels compared to the previous configurations which gave us 36000 discrete levels for duty cycle control. That's the meaning of and the motivation behind achieving higher PWM resolution.

STM32 PWM Resolution Control

As I've stated earlier, in order to control the PWM resolution, you'll have to give up on the F_PWM parameter. At least if you've got a strict PWM resolution requirement. Like having to have a 12-bit PWM output in order to match the ADC's resolution and directly map the ADC readings to the PWM duty cycle's output. In that case, it's fine as long as you don't care about the PWM frequency being at an exact value (1kHz, 10kHz, etc).

However, a more realistic situation is to have a PWM resolution requirement of at least (n bits) or more but the F_PWM has to be an exact value. In that case, you can easily use the PWM resolution control equation below and try plugging in the least acceptable resolution (n bits), and see if the F_PWM is overshooting or undershooting, and adjust the PSC or F_CLK (if possible).

For example, I once had a customer in the lighting units industry who had a strict requirement for the F_PWM = 20kHz. While also having to have at least 13 bits of resolution for each color PWM output channel (purely for competitive marketing purposes).

Let's use an STM32F103 @ 72MHz, plug the numbers into the design equation above, and see how it goes!

16 Bits = log₂(72M/20k(X+1))

By solving for X (which is the needed prescaler value), we'll get a -0.56 (negative value)

Now, it's impossible to meet the design requirements using that target STM32 microcontroller. Let's move to an STM32H750 @ 200MHz and re-run the calculations once more.

16 Bits = log₂(200M/20k(X+1))

By solving for X (the PSC value), we'll get a 0.22 (positive value), which means it's feasible to achieve the desired resolution at the F_PWM of 20kHz using that target microcontroller. And here is how to set it up:

By rounding down the PSC value, we'll get (PSC = 0).

Using the F_PWM equation, we'll get an ARR value of (ARR = 9999). To double-check the PWM resolution value at these settings, let's do it one last time:

Resolution_PWM = log₂(9999+1) = 13.288 [Bits], target achieved!

And there we have it! This combination of PSC & ARR values will help us achieve both requirements of F_PWM = 20kHz and PWM resolution of no less than 13 bits as desired.

STM32 PWM Frequency & Resolution Control Example

In this example, we'll configure the STM32 PWM Frequency & Resolution in such a way that meets the following requirements:

• Use STM32F103 (@72MHz)
• F_PWM = 10kHz
• PWM Resolution ≥ 12 Bits

We can proceed with the calculations in the same way as we did in the previous sections' examples. However, I've made it even easier for you, so you can just use our STM32 PWM Calculator Tool to figure out the configuration values needed to achieve the requirements above.

As you can see in the results above, our STM32F103 target MCU is able to achieve 12.8 bits of resolution while generating a PWM signal with an F_PWM = 10kHz. Therefore, you can just plug in the resulting (ARR & PSC) values in the CubeMX code configurator and you're good to go.

Sometimes you might need to precisely control the PWM resolution regardless of the output PWM's frequency (F_PWM). For example, if you need exactly 12 Bits of resolution to match an ADC's resolution and directly map the results from ADC->PWM. While having a non-strict requirement for the F_PWM ≥ 5kHz, for example.

In that case, you can use the other tab in the calculator tool (PWM Resolution Control), as shown below.

To achieve an exact PWM resolution of 12 Bits, the ARR value has to be exactly 4095. At these settings, the calculated PWM output frequency (F_PWM = 8.79kHz). Which is ≥ 5kHz as required, so it's fine to go ahead and use those (ARR & PSC) values in your CubeMX configurations.

Required Parts For STM32 Examples

All the example Code/LABs/Projects in this STM32 Series of Tutorials are done using the Dev boards & Electronic Parts Below:

★ Check The Links Below For The Full Course Kit List & LAB Test Equipment Required For Debugging ★

Wrap Up

In conclusion, we've explored the STM32 PWM Frequency & Resolution calculation and configuration. We've also discussed why it's important to have as high a PWM resolution as possible and how both the F_CLK and the F_PWM (PWM output frequency) dictate the maximum obtainable PWM resolution.

You can build on the provided examples and use this information in your system. To learn more about other features of the STM32 PWM, you can also check out the rest of the tutorials in this series.

(1) STM32 PWM Output Tutorial

(5) 3-Phase PWM (Center-Aligned)

(2) PWM Frequency & Resolution

(6) 3-Phase PWM (Edge-Aligned)

(3) PWM Input Mode

(7) STM32 PWM Phase Shift (Timer-Synchronized)

(4) Complementary PWM & Dead-Time

(8) STM32 PWM Shutdown (Break Input)