It wasn't easy but yes, I managed to solder a wire to that pin.
According to the datasheet: "Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period. This occurs only if the current through the winding is decreasing (per the indexer step table); if the current is increasing, then slow decay is used.”
It appears that mixed decay works better as the speed of the motor increases. For slow stepper speeds, fast decay may be needed and then as the motor speed increases mixed decay can be used, assuming that one has access to DECAY pin.
Slow decay benefits: Reduced noise, less ripple in current waveform resulting in slightly higher RMS currents (and thus torque) compared to fast decay.
Slow decay drawbacks: Increased thermal load on driver, can run into issues following target indexer waveforms based on certain L/R combinations, which means stepper position can momentarily be lost and/or steps be lost. As in the case above, microsteps became erratic and uneven.
Fast decay benefits: More precise positioning, especially at slower speeds, since stepper can be made to precisely conform to stepper index table. More forgiving thermal performance. A quick test with a heat gun, one can expect 15 to 20 degree increase in ambient temperature margin in fast mode versus slow mode before FAULT condition is reported. Slow decay at room temp with no motion faults every 10 to 15 seconds. Fast decay at 35C with no motion never faults.
Fast decay drawbacks: Higher ripple currents can lead to unwanted noise & slightly reduced overall torque due to reduced RMS currents.
Edited by yba2cuo3, 22 March 2021 - 09:57 AM.