Pinball: Debugging the PSoC Motor Driver

Summary

I spent a lot of time the last few days building and testing the Pinball board.  This morning when I plugged in my oscilloscope to test the PSoC motor driver, I saw this:

Here is a screenshot directly from the oscilloscope.  When I saw this, I wondered what was going on.  Why does the driver pull-up very quickly, but have some kind of RC delay on the pull-down?  This was particularly troubling as the PWM was set to 50 Hz (see on the picture where the (1) Freq is showing the measurement)

Things got much worse when I increased the frequency to 500 Hz.  In fact, the motor driver doesn’t pull-down at all.

PSoC Motor Driver

So where do I start debugging?  First I looked at the PSoC Creator schematic.  When you look at the schematic you can see that I took the PWM output and steered it to either M0L or M0R (or M1L or M1R).  Then I used a control register to flip it.

The M0R/M0L signals are used to control the switches on the H-Bridge.  Here is the actual schematic from the PCB:

I chose the Toshiba TB6612FNG motor driver chip for this design.  It has two H-Bridges built in and is the same one that we used on the PSoC 211 Robot.  You can see from the schematics above that I save 1 pin by attaching the PWM input to “H(igh)” and that I pulse either In1 or In2.

When you look at this table, or the schematic, you’ll see my problem.  I connected one of the inputs to “L”  That means that the NMOS switch that controls the pull-down is always off.  When the PWM is low, there is no pull-down.  That explains the funny RC pull-down, which is just some leakage on that node.

 

To fix this, I make this change to my PSoC Creator schematic.  In this configuration I get the following:

CR=00 the output = 00 (stop)

CR=02 the output = 1PWM = CCW

CR=03 the output = PWM1 = CW

Now when I run this thing, here is what I get (at my desired 20KHz):

And finally a video showing the PSoC Motor Driver working: