topic Re: tPM PWM Fluctuation& Resolution (FRMD-K66F) in Kinetis Microcontrollers

tPM PWM Fluctuation& Resolution (FRMD-K66F)

songja — Fri, 05 Apr 2019 12:13:08 GMT

Hello,

I am trying to send a PWM signal to a motor controller for torque control.

I have a couple of questions regarding tPM_PWM.

Firstly, the PWM signal is fluctuating within 0.5% around the target duty cycle when received at the motor controller.

My colleague has tried with Arduino to the motor controller and the duty cycle did not fluctuate.

I modified an example code(tpm_simple_pwm) to make it run with FreeRTOS.

But it still has the same structure for PWM.

The scheme is that when the SW button is pressed, it starts sending PWM to the motor controller.

Here is the code I used.

if (Motor_Switch==1)
{
/* Disable channel output before updating the dutycycle */
TPM_UpdateChnlEdgeLevelSelect(TPM_BASEADDR, (tpm_chnl_t)TPM_CHANNEL, 0U);

/* Update PWM duty cycle */
TPM_UpdatePwmDutycycle(TPM_BASEADDR, (tpm_chnl_t)TPM_CHANNEL, kTPM_CenterAlignedPwm,
updatedDutycycle);

   /* Start channel output with updated dutycycle */
   TPM_UpdateChnlEdgeLevelSelect(TPM_BASEADDR, (tpm_chnl_t)TPM_CHANNEL, TPM_LED_ON_LEVEL);
}
else
{
/* Disable channel output before updating the dutycycle */
   TPM_UpdateChnlEdgeLevelSelect(TPM_BASEADDR, (tpm_chnl_t)TPM_CHANNEL, 0U);
}

PWM frequency of the motor controller is 5kHz and TPM_SetupPwm was set to 24kHz.

I tried to match those two values (TPM PWM was set to 5kHz).

(TPM_SetupPwm(TPM_BASEADDR, &tpmParam, 1U, kTPM_CenterAlignedPwm, 5000U, TPM_SOURCE_CLOCK);)

Secondly, according to the example code, I can only send duty cycle from 0% to 99% with 1% increment.

I had a look at the underlying codes as well but I could not figure out how I can increase the resolution of the dutycycle.

Is there anywhere that I can change to increase the resolution of duty cycle?

Your advice will be very much appreciated.

Regards,

Re: tPM PWM Fluctuation& Resolution (FRMD-K66F)

AldoG — Wed, 10 Apr 2019 21:05:10 GMT

Hello,

You can indeed change the PWM frequency using the function you mentioned:

TPM_SetupPwm(TPM_BASEADDR, &tpmParam, 1U, kTPM_CenterAlignedPwm, 24000U, TPM_SOURCE_CLOCK)

You'll just need to change the 24000 to another value, for example in your case would be 5000.

Hope this helps,

Regards,
Aldo.

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Re: tPM PWM Fluctuation& Resolution (FRMD-K66F)

songja — Thu, 11 Apr 2019 07:19:23 GMT

Thanks for the reply.

I already changed it already to 5000U and the PWM signal at the receiving end still fluctuated +- 0.2%.

Also, do you know how I can change the resolution of PWM?

The example only seemed to work with every 1% of dutycycle. For example, it could not send 50.5% dutycycle.

I have looked into the underlying code but still had no clue to change the resolution for PWM.

Regards,
Jaeyong Song

Re: tPM PWM Fluctuation& Resolution (FRMD-K66F)

AldoG — Thu, 11 Apr 2019 15:56:16 GMT

Hello,

Yes you can change it but you'll need to change some settings in the TPM driver, go to the definition of the function TPM_UpdatePwmDutycycle, then you'll see the operation where the duty cycle is set cnv = (mod * dutyCyclePercent) / 100, change the 100 to a 1000, that way you'll have more resolution, for example you can achieve your 50.5%.

Also, make sure that you use an 16 bit int for the updatedDutycycle and for the pass parameter of the TPM_UpdatePwmDutycycle, since you'll use values from 1 to 999 (0.1% to 99.9%)

Hope this helps,

Regards,

Aldo.

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Re: tPM PWM Fluctuation& Resolution (FRMD-K66F)

songja — Fri, 12 Apr 2019 13:40:49 GMT

Hello,

It worked after changing the value from 100 to 1000 and also the pass parameter to uint16_t both in c and header files.

Thanks a lot.

Regards,