Dual MC33926 question

I want to verify my understanding of this controller.
I know that you can PWM either the in1/in2 pins or the d1/d2 pins but it seems that the using the d2 pin lets me obtain a lower speed (via a lower duty value) than with th in1/in2 pins. (I am using the 29:1 Metal Gearmotor 37Dx52L mm motors).

Is there an easy explanation why this is true?
Are there any other considerations in using d2 for PWM versus the in1/in2 pins?

To rotate in one direction, I let in1 float, take in2 low and PWM d2.
To rotate in the opposite direction, I let in2 float, take in1 low, and PWM d2.

What is the best way to brake in this scenario?

Thanks for any help!


First off, you should not be letting important inputs like the ones you are discussing float (the chip or board already has various pull-ups or pull-downs to provide defaults so that the pins are not really floating.)

I think the results you are seeing are caused by multiple factors, such as the frequency and duty cycle of your PWM. The basic difference between the two approaches you mention is that in one case, you are alternating between power and brake (both outputs driven low), and in the other case, you are alternating between power and coast (both outputs floating). If you want to brake in your scenario, you need to make both IN1 and IN2 low and then PWM d2 with the amount of braking you want.

- Jan


When I say I am letting in1 or in2 float, I am making MCU pins connected to them “inputs”.

I thought that is what the Truth Table for the MC33926 meant when under the “device state” column it shows conditions,status and outputs for IN1(or IN2) Disconnected (Z - High Impedance) in rows 7 and 8 of table…

(First off,) I got told that my “First off” above sounded really hostile, which was certainly not my intent; I use it as an introductory phrase for things that I feel need to get covered and out of the way but are not the meat of the discussion. So, sorry.

Anyway, the table is showing you what happens in various conditions including bad ones (e.g. inputs disconnected); that does not mean you should intentionally make those states happen. The datasheet says (on page 3) that IN1 and IN2 are internally pulled up, so I am also not sure how much I trust that table. Is there a reason you are making your MCU pin an input instead of driving high?

- Jan

I didn’t take offense at “First off”…

I thought that I had tried in1 high and in2 low and then PWM d2 without success but it seems to work fine now. The spec sheet for this chip is a little confusing/complex and I was trying various approaches.

I have a better handle on it now, but I need to experiment more.

Thanks for your help.
- Ron