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Here is the microstep library: http://printplussupport.be/microstep.html
Had a similar issue with powering a stepper on one of these. I ended up using a microstepping library I found online and modifying it slightly. I noticed the device would reset and have a kinds of crazy errors if I tried to do more than 30 steps at a time. So I added a delay(1) to the code every 30 steps. This has 100% resolved issues with resets and errors driving steppers from the esp8266 chipset.