Mixed decay in full-step mode - impossible?

Hi Pololu,

My company is considering switching to your A4988 driver modules, but one question before we do regarding decay mode…

We want to operate our Nema-14s in mixed-decay mode, full-step. I see in other forum answers it’s mentioned that the Pololu boards operate in mixed-decay mode with t(off) = 12us re the 10k resistor connected between ROSC/gnd, but per the A4988 datasheet with the setup " ROSC through a resistor to ground — off-time is determined by the following formula; the decay mode is automatic Mixed for all step modes except full-step which is set to Slow" so am I right in saying that the Pololu boards only operate in mixed-decay mode for all step modes except full step and would be operating in slow decay for full-step?


Yes, the A4988 driver only uses slow decay when the driver is in full-step mode (and mixed decay with 12us t(off) on our carrier board, otherwise). If slow decay mode is not controlling the current well in your motors and you want to use full step mode, you might look at some of our other driver carrier boards like the DRV8880, MP6500, or TB67S279FTG carrier boards, which generally have more sophisticated current control algorithms.


Thanks Nathan, we’ll take a look at the others.

Just out of interest, why the 10k resistor? Is there something advantageous about a 12us t(off) over the default 30us if ROSC connected to gnd?


A shorter t(off) increases the PWM frequency, which can help maintain a more steady current in low inductance systems, and the ROSC resistor also sets the driver to use automatically-selected mixed decay, which the datasheet generally recommends. Grounding ROSC sets the driver to use 100% mixed decay, which seems to be a more specialized setting that introduces more ripple current but reduces the chances for missed microsteps under certain conditions.

By the way, we always have the option of doing a custom run of our driver boards for you with a 0 Ω ROSC resistor instead of the normal 10k one. Please email us if that is something you would want to pursue further.