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If you're using a DRV8825, Pololu has a good tutorial and library on getting a stepper motor rotating.
https://www.pololu.com/product/2133/resources
https://github.com/laurb9/StepperDriver

/*==========================================================================
* This is a minimal sketch showing the usage of TeensyStep
*
* STEP Pulses on Pin 2 (can be any pin)
* DIR ...

STOP writing code that uses 'delay'. that included the microdelay... (unless you REALLY know what you are doing).

You are setting yourself up for a world of hurt otherwise.

I am far from...

The servo tester works when connected to the ESC because (likely) the ESC has a BEC (battery elimination circuit) -- which is basically a voltage regulator to step down the battery voltage (which is...