A4988 Missing Steps - Mixed Decay Mode

I’m using an A4988 Pololu board to drive a telescope tracking unit. In 1/16 microstepping this is roughly 17 pulses for second, quite slow.

The drive is very ‘lumpy’ and it seems I’m suffering missing steps as shown on P.8 of the Allegro datasheet.

I have shorted out the resistor on ROSC to short it to ground which means the mixed decay mode is on all the time. This SHOULD solve the issue, but unfortunately it makes thing BETTER but it doesn’t get rid of it. I need the drive to be as smooth as possible to minimise vibration.

I have tried changing he drive current and the drive seems to be smoothest at about 350mA (about right for this application) and it gets worse both above or significantly below this current,

I can see threads where this has been addressed successfully with other boards, has anyone cracked it with an A4988?


What supply voltage are you using for your motors and what are the voltage and current rating of your stepper motor coils? If you do not have the voltage and current rating, the resistance of the motor coils (which can be measured with a multimeter) would be helpful.


Hello Nathan,

Sorry, I missed the notification until just now.

12v supply, I THINK it’s a 24V 2A stepper, 5.5 ohms, but I’m running it at about 350mA as this is far more torque than needed for my application (telescope drive).

If your coil resistance is 5.5 ohm and the rated current is 2 A, then the rated voltage is probably 12 V, not 24 V. With the current limit set to 350mA, a pair of 5.5 ohm coils should only need about 2V to be driven, so using a lower power supply voltage would be the most straightforward way to help your problem, but the A4988 only works down to 8V.

This thread on our forum discusses a similar problem and might be helpful for you. Using a different driver that offers a full-time fast-decay mode might help you get a smoother motion. Our new DRV8880 carrier board has an AutoTune feature that automatically selects the decay mode every PWM cycle in an attempt to optimize current regulation, and it can be placed into full-time fast-decay mode (or one of eight other decay mode configurations) if that does not work.

Also, this blog post discusses using diodes in series with the stepper motor to raise the threshold voltage at which current flows, which seems to have worked OK for them and seemed to help when I did some testing here.


Thanks Nathan

I’ll try taht diode fix - it’s worth experimenting.

how to ground ROSC pin ?
mine a4988 clone has 0 ohm R4 resistor