A4988 Spins motor with no step signal

I have a A4988 Stepper Motor Driver Carrier with Voltage Regulator on a breadboard. It is connected to a 12 V switching power supply and a 4 lead bipolar steeper motor. I followed the wiring diagram posted on the page of this driver. Also I added a 100 uf capacitor between the power lead of the vmot input.

When nothing else is connected to the carrier, the motor spins. There is nothing connected to the step or dir lines. Is this a normal behavior?

if I ground the step and dir, or if I connect the driver to an Arduino, then it stops. it seems that it is responding ok to the micro controller, but some times I get some erratic behavior, like stalling the motor for a second or two when dir signal is changed.

So, should the motor spin with no signal on the step input? I’m just wondering if I might have made a wiring mistake

Thanks in advance

It is a bad idea to leave an input unconnected, unless its state is determined by a “pullup” or “pulldown” resistor. What you are observing in this case is typical of what can happen.

I found Stepper motor moving automatically that has a similar problem to mine. According to Ben ( from Pololu), the solution is:

One is to connect your microcontroller’s reset line to the reset line on the stepper motor driver (or you could invert the signal and connect it to the enable line). Another is to include an external pull-down to ground (or pull-up to VCC) on the step pin so that it is pulled to a fixed voltage when the pin disconnected or the microcontroller input is floating

I will be using for my final design the DRV8825. Regarding this carrier, for what I understand in the spec sheet, the step, dir and ms0-2 lines are internally pull down. Do I have to provide external resistors in order to avoid surprises on the drv8825.


If the spec sheet says the pulldown resistors are there, you should not need them. But check to be sure!

Thanks Jim. I’ll check it out. The only reason why I was avoiding the resistors was to save space on the pcb design.