Power fault with AVR128DB Curiosity Nano and Apple IIgs minimal circuit, possible parasitic powering

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I'm seeing an unexpected power fault with a very simple circuit involving the AVR128DB Curiosity Nano and an Apple IIgs. Does anybody see an explanation I might have overlooked?

 

  • Curiosity Nano powered from USB, with built-in debugger configured to supply +5V to the target
  • GND on the Nano connected to GND on the Apple IIgs disk port.
  • PD0 on the Nano connected through a 1.5 meg resistor to RD input on the IIgs disk port. Inside the IIgs, RD has ~10K ohm pull-up to +5V, and is connected to an IWM chip input.

 

Originally there was no resistor, then I inserted larger and larger resistors in an attempt to understand this problem.

 

When I turn on the Apple IIgs, and then connect USB power to the Nano, the Nano's green LED flashes continuously. The manual says means there's a power fault. If I disconnect PD0 and repeat the same steps, no power fault. The program I'm running is irrelevant, and I see the same behavior when I replace my entire program with a do-nothing main() function.

 

I'm suspicious that the AVR128DB on the Nano is being parasitically powered from the IIgs, through a tiny current from the 5V 10K pull-up in series with my 1.5 meg resistor and into PD0, and this somehow confuses the debugger's voltage regulator when it turns on. If I measure the Nano's VTG pin (target voltage) with the IIgs on but the Nano's USB power off, I see 0.65V rather than 0V.

 

Does this make sense? If so, any good way I can work around this? If I turn on the Nano's USB power before the IIgs, then it will apply signals to unpowered chips inside the IIgs, and that may be bad. Can I power the AVR128DB from the Apple IIgs instead of the debugger, without losing the ability to debug?

 

 

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I would connect a resistor from PD0 to ground. Size it to match the 10k+series resistor so the divided voltage from the Apple is too low to power the AVR. It should be high enough resistance that the AVR can drive the pin without struggling.

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Whoops, I needed to do a better job of reading section 4.3 of the Curiosity Nano manual, where the power supply operation is explained. If the debugger senses an externally-applied target voltage like I'm getting from parasitic power, it will signal a power fault. The solution is to tie the /VOFF pin low to disable the debugger's target voltage regulator, and power the target from the Apple IIgs instead. The debugger still works fine with this setup. 

 

In brief: pay more attention when using an externally-powered device with the Curiosity Nano and its built-in target voltage regulator.

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GND on the Nano connected to GND on the Apple IIgs disk port.

While that is both highly desirable and necessary, it could also be the cause of many problem, or even damage.

 

In order to connect the gnds of separate devices together they must be at the exact same level, or one must be floatable with respect to the other, so they can equalize.  

Say neither one was floating and they each formed a hard path to earth gnd, but with a minor voltage difference between them (before typing them together). So, now you are connecting together two that have a slight mismatch, and since neither can float, they must arm-wrestle it out using current flow to produce voltage drops, until a match occurs.  These unexpected arm wrestling matches can damage boards, scope probes, and/or lead to improper operation.

This can be especially severe in areas like audio/video, leading to howls and reruns of Geraldo.

 

Items like a USB isolator can do wonders to break these gnd loops & lessen the hassle of trying not to create them in the first place.

https://www.amazon.com/Isolator-...

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!