MCF8315A: Resolution vs. PWM input frequency

Hi Hideki-san,

We should have hysteresis for the frequency thresholds (need to check for the exact numbers) but can you help me understand why the motor speed varying is bigger than PWM frequency varying? 

Regards,

Vishnu

Hi Vishnu,

Sorry I was confusing you.

Please see below rough sketch. My understanding of resolution is described in the center. Since "bit" is an integer value, Res_pwm should be in a range of green. If there are hysteresis, please see the up sketch as an example, the resolution bit would be 10 bit or 9 bit when 60kHz. Duty @ 10bit and duty @ 9bit will differ a little. Customer concerns this varying. Can you please teach me where I am wrong?

Regards,

Hayashi

Hideki-san,

I believe the firmware should take care of the duty cycle when the resolution switches from 10 bit to 9 bit. We shouldn't be seeign any speed variations.

Regards,

Vishnu

Hideki Hayashi1 said:

2. How is the input frequency of the condition determined? (For example, determined by two periods of input PWM)

Hi,

For me I would use the speed input PWM=350hz as this will give the highest 14bit resolution for a fine speed control. 350Hz speed command update rate is pretty fast, so I don't understand who would want to use pwm = 100Khz with low resolution, and for what reason.

Why care about the hysteresis or not, as once you decide what pwm frequency to use, there is no good reason to vary it during the operation. 

Brian

Hi Vishnu Balaraj,

Vishnu Balaraj said:

I believe the firmware should take care of the duty cycle when the resolution switches from 10 bit to 9 bit. We shouldn't be seeign any speed variations.

For example in decimal number, 52.33% is 4 digits and 52.334% is 5 digits if the analog value is 52.33432...... like that. Actually the difference of the LSB of 10bits is very small, but the customer's concern is here. Did you mean this will not be seen?

Hi Brian Dang,

Brian Dang said:

350Hz speed command update rate is pretty fast, so I don't understand who would want to use pwm = 100Khz with low resolution, and for what reason.

No, actually I don't know the customer's PWM frequency right now. I just picked up as an example.

Brian Dang said:

Why care about the hysteresis or not, as once you decide what pwm frequency to use, there is no good reason to vary it during the operation. 

Customer definitely uses a single PWM frequency applied in SPEED pin. The concerns that the PWM frequency varies a little caused by any noise in the circuit. We should suggest customer not to use PWM frequency around the threshold?

Best regards,

Hayashi

Hi Hideki-san,

Speed loop accuracy in MCF8316A and MCF8315A is 3% with internal clock which will be higher compared to the error seen due to this bit resolution. In other words, bit resolution error will be masked by the 3% speed loop accuracy. 

Regards,

Vishnu

Hi Vishnu,

If so, there are lower bits are an output resolution 3% is nearly equol 1/32, so there is only 5bit as effective bit...

By the way, How the frequency value is detected and/or decoded? If customer wants to use 380Hz, but to think a stiatuion which MCF8315 accidentaly detect as 350Hz (Threshold value), there still be risks of the number of bits changes. Customer have to know it to take care of PWM generation.

Best regards.

Hayashi

Hideki Hayashi1 said:

Customer definitely uses a single PWM frequency applied in SPEED pin. The concerns that the PWM frequency varies a little caused by any noise in the circuit. We should suggest customer not to use PWM frequency around the threshold?

Hi,

I don't think you should concern about the frequency threshold point. I don't think the pwm duty cycle will be jumping around if the selected pwm frequency is at the "threshold point". In fact I don't think there is a discrete threshold point at all. 

Brian

Hideki-San,

3% error in speed loop is due to the drift in internal clock. Frequency of PWM is detected by measuring the time period between consecutive rising edges of PWM. Is the speed configured to frequency mode or duty cycle mode ? 

Regards,

Vishnu