Bench supply vs. chassis supply problem

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I'm building a system based on an ATMega128A.  My board (which only draws about 25 mA) works fine when running off my bench supply's 5V.  When I switch to the chassis supply I'll be using in the finished product (a Mean Well RPS-400-24-TF), I can program/verify the MCU, but it doesn't run the program nor will the emulator work.  The Mean Well is primarily a 24V supply, but it also has a 5V/1A standby output which I'd like to I use to power my board.  This is what seems to be causing the problem.  It produces an occasional 300 mV spike, but that's the only anomaly I can find, and that's well within the tolerance of the MCU.  What else should I be looking for?  Thanks for any tips.

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lautman wrote:
It produces an occasional 300 mV spike, but that's the only anomaly I can find, and that's well within the tolerance of the MCU.
Only 200mV greater than the BOD lower limit.

Excessive VCC dv/dt will affect the internal oscillators and the crystal oscillators (jitter (phase noise) is proportional to power supply noise)

mega128A's reset threshold is healthy but its reset pull-up is a bit weak; there's a connection between VCC and the reset signal (reset pull-up current source)

lautman wrote:
else should I be looking for?

  • Chassis supply datasheet, 5V/1A standby output, is there a LC filter to the load?
  • Ferrite bead between chassis supply and mega128A VCC
  • Reset signal

 


ATmega128A - 8-bit AVR Microcontrollers - Microcontrollers and Processors

AVR042: AVR Hardware Design Considerations

 

edit: reset

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Wed. Dec 12, 2018 - 06:36 AM
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Power supplies often have a minimum current needed to be drawn to work properly. If you have nothing on the 24V output then this might be your problem.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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A small inductor or choke in series with the power supply forms a filter with the already mounted buffer and decoupling capactiors for your atmega.

 

Euhmm..

You do have bypass and buffer cap's on your uC board I hope?

Even with a good schematic, bad PCB design can be a cause of unreliable operation.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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I can program/verify the MCU, but it doesn't run the program

Now that is indeed very odd, as the uC IS in fact running when one programs it.

 

Anything else changed between the two setups?

 

Will the uC run properly if you power if from a battery, but have it placed right next to the problematic power supply?

 

Will the uC run properly if you power it from a battery, but have it right next to the problematic power supply and they share a common ground?

 

Can you put some other load on the problematic power supply's 5V output, and then power up the uC, does that then work?

(With some significant load, 100 - 200 mA for example)

 

 I think you need a good O'scope to watch the problematic power supply's output.

 

As already mentioned above you may well benefit from putting an LC filter on the incoming 5V rail to the uC.

 

JC

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DocJC wrote:
Now that is indeed very odd, as the uC IS in fact running when one programs it.
NVMC yes, CPU no

7. At the end of the programming session, RESET can be set high to commence normal operation.

Is RESET being completely de-asserted?

 

"Dare to be naïve." - Buckminster Fuller

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Ah, good point.

Keep me honest...

 

JC