DRV8308: Fclkin and Tach Output do Not Match

Reif Heck11

Intellectual 700 points

Part Number: DRV8308
Other Parts Discussed in Thread: DRV8307

Hi,

I have the DRV8308 running my 4 pole,24 volt motor with a maximum RPM of 10000. I have the following set:

PWM frequency = 25 KHZ and duty cycle of 50%
Set for Clock Frequency Mode.
I've set the registers as follows (Register dump from my custom board):
- MCR00[0xD901]
- MCR01[0x0000]
- MCR02[0x03B4]
- MCR03[0x6F82]
- MCR04[0x0340]
- MCR05[0x3000]
- MCR06[0x0481]
- MCR07[0x03BA]
- MCR08[0x012C]
- MCR09[0x0250]
- MCR0A[0x0000]
- MCR0B[0x0168]
Fault Register reads on power up:
- MCR2A[0x0018]

I have both LOOPGAIN and SPEEDGAIN Set to 0 for testing.

I am running the motor at 3600 RPM. I have to set the SPEED Register to 360 to get the motor RPM to 3600.

The motor runs smoothly. @90 mA.

The Tach output is bouncing between 3549 and 3584 RPMs ( I obviously need to tune it).

I have my own diagnostics window for modifying the registers, setting direction, applying the brake, enable, etc...

I am confused about how Fclkin and the Tach could ever be the same.

Is Fclkin supposed to be the PWM output from the ATMEGA 2560 timer? PWM frequency is 25 KHZ. How would that match 3600 RPM?

I'm going by the tuning guide to set these things up and trying to calculate the LOOPGAIN and SPEEDGAIN.

Can you shed some light on this?

Thank you,

Reif Heck

Finna Sensors

over 6 years ago

0 Adam Sidelsky over 6 years ago

TI__Guru 65190 points

Reif,

Per section 7.4.1 of the datasheet: "Clock Frequency Mode: This is closed-loop speed control that locks the FGOUT frequency with the CLKIN frequency."

FCLKIN is the frequency of your input clock.

When you mention that your TACH output is ~3600, what exactly do you mean? What are you seeing on the FGOUT pin?

Regards,

-Adam

0 Reif Heck11 over 6 years ago in reply to Adam Sidelsky

Intellectual 700 points

Hi Adam,

Long time no talk to ;~)

Anyway, my PWM clock frequency is 25 KHZ.

I am seeing a TACH period of 17 ms on FGout. Converting to RPM gives you ~3600 RPM. Right?

Else I am really confused ;~(

I've attached analyzer output for FGout and Fclkin (PWM?)

Reif

0 Reif Heck11 over 6 years ago in reply to Adam Sidelsky

Intellectual 700 points

Adam,

Is the FCLKIN supposed to be my PWM signal that I'm generating from the timer in my processor?

Reif

0 Reif Heck11 over 6 years ago in reply to Reif Heck11

Intellectual 700 points

I'll take an answer from anyone.

ANYONE?

Reif Heck

0 Adam Sidelsky over 6 years ago in reply to Reif Heck11

TI__Guru 65190 points

Reif,

FCLKIN is simply the Frequency of the CLKIN pin which is your clock signal input which dictates the output speed. This signal comes from your Micro.

How are you relating the FGOUT frequency to the RPM? The FGOUT is an electrical frequency related to the hall inputs (this changes based on your configuration) The device does not know your number of poles and therefore it can't know the RPM. The FGOUT signal is locked to the CLKIN frequency so that's why they correlate.

Regards,

-Adam

0 Reif Heck11 over 6 years ago in reply to Adam Sidelsky

Intellectual 700 points

Adam,

My clock signal from the micro is PWM set at 25 KHZ with adjustable duty cycle. So you are saying the FGout is not my tachometer signal? So where do I read RPM?

Also, if I change the PWM frequency, that should change the speed?

Thanks,

Reif

0 Reif Heck11 over 6 years ago in reply to Reif Heck11

Intellectual 700 points

Adam,

I read the user guide again and i see where in clock frequency mode RPM = (fclkin/2) * 60.

My problem is that I'm generating a 25 KHZ PWM clock which was what I had to use for the DRV8307. So in the user's guide they are using a 100 HZ fclkin.

So it looks like I need to bring the PWM frequency down to 100 HZ . Does that seem right?

Thanks Adam,

Reif

0 Adam Sidelsky over 6 years ago in reply to Reif Heck11

TI__Guru 65190 points

Reif.

In Clock frequency mode, the frequency of the clock controls the speed of the motor. If you change the clock frequency you should see this in the output.

Regards,

-Adam

0 Reif Heck11 over 6 years ago in reply to Adam Sidelsky

Intellectual 700 points

Thanks Adam.

I should have found the user's guide sooner. I see how this is supposed to work now.

So the 25 kHz PWM Frequency is now generated by the chip internally instead of having to be supplied (as was the case with the DRV8307) by the processor.

I was also confusing the FGout with the Tachometer Out (again from the DRV8307).

I think I have enough to get going.

Thanks again,

Reif

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DRV8308: Fclkin and Tach Output do Not Match