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Non-detected Stall L6474

Question asked by mouser on Oct 20, 2015
I am working  a bipolar stepper motor against a spring to assess the torque delivered. The test drives a set distance and then returns. Software produces a constant accel profile in incremental steps of pulse rate. Most of travel is a constant speed ( in the region of 2kHz pulse freq ).

At some stage the motor skips steps when the spring tension passes the torque it can deliver. This is the expected test result.

Usually this will result in some lost steps and the motor return but looses positional accuracy.

However sometimes the L6474 drops out and the motor falls back under the force of the spring. The return motion is not occurring and the output is powered off.

I have a hardware interrupt on the FLAG pin that will report an error and interrogate the chips STATUS register for a diagnostic.

This firmware is tested and will report any errors, in particular, a THWRN which would seem the most likely here. The red LED does not light up and no change in FLAG is detected.

What conditions can cause the output to shut off that is not indicated by the FLAG line?