Can FreeRTOS software timer generate PWM?

[HMSEdinburge]

Hello, i am using F446 and i am contemplating using FreeRTOS CMSIS V2 software timer to generate PWM to control motors via motor drivers (IC  chipset). Is this possible? Or must I use the hardware timer, TIM1~7 ?

[Andrew Neil]

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@HMSEdinburge wrote:

must I use the hardware timer, TIM1~7 ?

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Why would you not want to use them? This is exactly what they're for !

[HMSEdinburge]

Ok, but can this FreeRTOS software timer produce PWM like the hardware timer? Can they driver a motor? 

Please share source code of software timer generating PWM, thx. 

[Andrew Neil]

Of course it can be done - just like bit-banging any other interface.

But you will never reach anywhere near the performance of a dedicated hardware timer.

Your code will need to manually make every single output transition and timer adjustment - and fit this around whatever else your code is doing.

 

But it's just software; you can easily try it out - but pay particular attention to the CPU usage ...

[LCE]

Then check if the hardware timers are really needed for the stuff they are doing now.

If your PWM is really slow (like <= 1 kHz carrier or so), then maybe software can handle this well enough.

Otherwise - not at least a 10 kHz interrupt available or so?

But all this doesn't really make sense in a device that has hardware / timers built for that purpose.

 

[HMSEdinburge]

all my hardware timer TIM1~7 all used, therefore only left software timer, but can software timer able to drive an external motor? 

 

[Andrew Neil]

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@HMSEdinburge wrote:

 can software timer able to drive an external motor? 

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A timer - whether software or hardware - just toggles an output pin.

It makes no difference to the timer (whether SW or HW) what is externally connected to that pin.

 

As @LCE said, the limitation on using SW timers is going to be the maximum speed achievable.

Also the CPU load.

 

But, again, you can easily just try it out: write some code to toggle a pin, watch it on your scope, check CPU utilisation...

[Gyessine]

Hello @HMSEdinburge 
Well, it's doable but as @Andrew Neil and @LCE but do not expect high precision. Software timers can be used for motor control in low-precision or low-frequency applications. However, they are not a substitute for hardware timers when precise timing is required.

If the software timer you want to create have the same frequency and period as one of the hardware timers, you can create a variable that captures the hardware timer CNT register. Compare this variable with specified value (like a custom ARR) as a condition to perform the required task when the CNT register reaches the desired value for the software timer. This approach may improve precision slightly.

BR
Gyessine

[mƎALLEm]

As said by @Andrew Neil ,

There are 14 timers in STM32F446:

You are using half of timers resources in your case only 1 to 7 in your case.. So why bothering yourself with the software timer usage?

[HMSEdinburge]

Hi there, i got total 63 motors to control. F446 has 14 timer, TIM6 and TIM7 is a general timer, does not have CCR and AAR registers, so cant use for PWM. 

Anyone has the source code for software timer using FreeRTOS CMSIS V

[mƎALLEm]

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@HMSEdinburge wrote:

Hi there, i got total 63 motors to control. 

Anyone has the source code for software timer using FreeRTOS CMSIS V

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63 motors to be controlled with STM32F446 with FreeRTOS software timer to generate PWM?

I don't think this is feasible. Think about the CPU load and the accuracy of the PWM duty cycle!

This is something that needs to be done with hardware ..

[AScha.3]

Hi,

to get good info, you should also give some good info:

- how many motors need control at same time ?

- which kind of motor and driver ? (DC, stepper, 3-phase ...etc)

- needed speed of pwm or control signal ?

- and resolution of pwm needed (how many steps from 0...100% ) ?

 

[HMSEdinburge]

These are the motors i m using, BLDC, stepper, servo actuators, stepper motor need 1/32 steps, BLDC needs PWM, and their various motor drivers, gate drivers etc. Total 63 motors. 

[AScha.3]

So to answer your question:

>Can FreeRTOS software timer generate PWM?

yes.

+

can you control 64 motors of different kind in real time just by using some RTOS:

no.

[unsigned_char_array]

• Software PWM is limited in frequency/resolution. If you want 0-100 steps and use a 1 ms tick your frequency will be 10 Hz. 
• You can share timers and handle things in software. But this results in dither as interrupt latency is not always predictable. Also can be a high load to CPU
• Best is to use hardware timer
• second best is to use external PWM IC. For example a SPI or I2C PWM IC. Such as the 16 channel PCA9685. Note that the PCA9685 is optimized for LEDs, but you can can connect it to a transistor switching a DC motor. Or to the enable pin of an H-bridge and use software to switch direction.

 

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@HMSEdinburge wrote:

These are the motors i m using, BLDC, stepper, servo actuators, stepper motor need 1/32 steps, BLDC needs PWM, and their various motor drivers, gate drivers etc. Total 63 motors.

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• BLDC needs 3 phase control synchronized with back-emf detection. An MCU is not powerful enough to do several of these. Get a dedicated motor control IC/complete motor driver for each BLDC motor.
• Get a dedicated motor control IC/complete motor driver for each stepper motor.
• If they are RC servos you can use the PCA9685 for the 1-2ms pulse

 

63 motors seems like a lot. If the MCU has to do additional processing it may be underpowered. It's also rather ambitious for a beginner such as yourself. What is the application?