A4988 erratic when cold

Hi all,
we bought a Pololu A4988 stepper driver to drive a 1.8 degree/200 steps per rotation steppermotor.
The plan is to drive the stepper with 1/16th microstepping, so we fixed a stripboard with all the necessary for the driver.
Enable connected to the ground, the MS1,2 and 3 pulled up with a 100 ohm resistor at 5V, connected Sleep and Reset with eachother, provided a 5V for VDD-GND and a 12V for VMOT-GND with a 100 uF cap on the board in parallel.
And then calibrated the current by setting VREF on the potentiometer to 0,4 V.

Now the problem:
When powering up from a cold start (ambient room temperature) the stepper behaves erratic with uncoordinated stepping and backstepping without a single pulse set on the STEP input.
Then we sent some low-high transitions on the STEP input with DIR high and the erratic behaviour somethimes gets interrupted by some dirty rotation.
After a minute or two when the chip has warmed itself up a bit, the erratic ticking decreases and the steppermotor runs smooth with only now and then a strange tick.

Anyone an idea what might be our problem?

Thanks for your time,

Tim and Kevin

Hello, Tim and Kevin.

That definitely seems like strange behavior. What are you controlling the driver with? Are the STEP and DIR pins always being driven either high or low? If they are ever in a floating state, it could lead to the erratic behavior you describe.

Another explanation might be that there is a intermittent solder connection somewhere; could you check that you have good connectivity on the solder joints between the board and the header pins, and inspect the surface-mount components on the board for any bad joints?

- Kevin

Hi Kevin,
thanks for the fast reply.
We’ve been checking the solder joints and they seem all pretty solid, so I think that cause can be checked off.
The board is controlled with an Atmel Atmega 644P and the STEP/DIR are always either in a high or in a low state, the STEP normally ends low after steps but somethimes he remains high after emitting the step transitions. Could this cause problems?
We’ve checked this with a scope, and when behaving erratic, there’s no abnormal signal or noise on the STEP input.
One thing where I’m not sure about, is the necessity of a capacitor (of around 50 uF) in parallel with the 5V, we haven’t fitted this one on our stripboard, but we have one across the 12V (100 uF). We’ve also checked the 5V with the scope during erratic and normal operation and no noticeable voltage dips or noise.

Grtz, Tim

Do you have a good ground connection between your controller and the stepper driver? Could you post a picture or diagram of your setup showing how you have everything connected?

Also, you mentioned before that you have the microstep inputs pulled up with 100 ohm resistors; have you verified this? On the off chance that you are actually using 100k resistors, that could be putting those inputs into an undefined state.

- Kevin

Jup the ground connection between controller and driver is a solid copper wire.
I’ve also checked the pull-up resistor from the microstep inputs and it is a 100 ohm, so those should be okay.

This is the setup we’ve replicated:

Grtz, Tim

It looks like those pictures are from the RepRap wiki, so if you are sure your setup matches them, it should be fine. Unfortunately, I am not sure why your driver is not working properly. If you contact us directly with your order information, though, I will see what I can do about getting you a replacement.

- Kevin