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Why do my ICs keep dying when scope probing?
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Mike Ro:
I am experimenting with TMC2130 silentstep stick modules in SPI mode and a NodeMCU.

Mostly, I have everything working fine. But occasionally either the TMC2130 or the NodeMCU fails.

The failure modes are varied, the most common is that the TMC2130 responds to SPI, but does not even attempt to drive the motor (SPI reports open load). I have another TMC2130 which continuously draws 350mA even without a motor connected and gets really hot. Also, one NodeMCU failed with the GPIO pin (connected to STEP) no longer driving properly.

These failures seem to occur when I have been probing the signal (usually STEP or SPI) whilst the USB is connected to my PC.

Instinctively, this sounds like a earth problem (bench power supply outputs are floating, USB is floating, scope ground is at earth potential).

However, I have checked and double checked that that all the connections are properly tied to 0V. I have a single, low impedance 0V track that connects the NodeMCU ground (PC USB shell potential), TMC2130 driver motor power ground (bench supply - output) and scope ground (mains earth). The maximum DC voltage between any point is around 28mV and AC voltage is negligible.

I am only driving the stepper motor at 300mA (both driver and TMC2130 are rated at over 1A).

Normally, the circuit can run for days without problems, but every now and then I tweak the code and need to probe the circuit. This is when the problems occur.

I feel like I am missing something really obvious, but what?



garethw:
Do you have sufficient bypass capacitors on the power pin of the IC?


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Mike Ro:

--- Quote from: garethw on June 10, 2019, 11:00:08 am ---Do you have sufficient bypass capacitors on the power pin of the IC?

--- End quote ---

Thanks, that's a good point, but I think it is okay - the schematic for the module is here:

https://github.com/watterott/SilentStepStick/blob/master/hardware/SilentStepStick-TMC2130_v11.pdf

There are bypass on both the VM and VCC_IO lines.
Gyro:

--- Quote from: Mike Ro on June 10, 2019, 10:58:28 am ---Instinctively, this sounds like a earth problem (bench power supply outputs are floating, USB is floating, scope ground is at earth potential).

--- End quote ---

Welcome to the forum.

My instincts would be the same as yours, but it sounds as if you have all the bases covered.

A couple of random thoughts...

- Is the PC grounded? A possibility if it's a laptop with a 2 pin mains lead (Y-Cap leakage).
- You mention USB shell potential, that's not necessarily the same as the USB 0V signal. The two are normally only linked at one end, might relate to the above.

It sounds like the occurrences are very tied to the times that USB is connected, so logically it's probably either the interface itself or what's connected at the other end.


P.S. I would also look at more bulk capacitance on the TMC2130 supply. The internal output clamp diodes need something to shunt back-EMF from the motor into without spiking the supply too much. I have no idea how this would be USB related though.
DDunfield:
It does smell like a grounding problem. You've checked that no measurable current flows between scope ground and device ground?

Assuming you've got a fairly standard scope with 1meg inputs, and are using a 10x probe, then touching a point on the circuit with the probe should be akin to touching it with 10 meg resistor to ground. which I would not think would be significant.

A couple of other things to consider.

The scope probe will present a slight capacitance compared to a pure resistor, but again I would not expect it to be significant (It's very low).

The probe may be injecting noise into the circuit. Could be through grounding loops, could be something is acting like an antenna, hard to say.
I don't have experience with the module in question, however I note that it is an "H-bridge" which are unhappy of both branches get turned on at the same time (even briefly).
It could be that noise is getting injected and causing the H-bridge to enter a "funny" state, possibly enabling both sides at once, big spikes, potential lockup etc.

Your module drawing significant current even if no motor is connected smells of a H-bridge failure.

Dave
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