Scope Probe loading

Merry Christmas and Happy New Year.  I hope your holidays are filled with friends and of course blinky things.


I posted on Espressif’s forum under bugs for a feature request. At the time that I write this post, there have been 26 views of the request.  I hope it gets enough interest that Espressif considers my request.


I soldered a 2 pin header into pins 1 and 2 of the programming connector. I then connected 3.3 Volts to Pin1.  I hooked up my Oscilloscope to pin 4 and immediately saw data on the line.  Pin 4 is attached to MISO.  This is great news, the data is getting through the level shifter.  Unfortunately, the waveform shows a lot of noise.  Then I switched to 5V and I saw data again, but the firmware crashed very quickly.  I started out by assuming the 5 Volts was the problem. So I tried setting it back to 3.3 Volts and the system wouldn’t reboot.  Then I disconnected the voltage supply and still not booting.  Finally, I disconnected my oscilloscope probe and it would boot again.


The MISO line brings the code from flash into the processor. Any glitch on it could cause crashes/failures.  It appears that the scope probe added to the output of the chip was enough to cause a glitch back on the MISO line.  This is disappointing, scope probes are very minimal loads.  Typically 10pF and 10MΩ on the circuit.  That is less than most devices that I would want to program.  This is going to take some more time to look at.  I need to test with a larger load, and maybe some pullups/pulldowns.  I am beginning to think a port expander might be a better choice. It’s still early to make that decision.


I started writing this post on Saturday before Christmas.  I didn’t get much done then.  I don’t expect to get back to it over new years weekend.  So this is the only post for both weeks.  I hope you get some value reading this blog.

Next Year:

I am considering the value of this design as it stands.  The Raspberry Pi might be a better platform to achieve this goal.  Put on a small board that does the level conversion, and generated the programming voltage might be a new direction for the design.  The Raspberry Pi has a lot more IO available and more power and RAM. On the other hand, it could turn into a more expensive project because I would want to add an external ADC.

What do you think?  Would I attract more software developers if I used the Pi?  Is there a direction you would like this project to go?


One thought on “Scope Probe loading

  1. Pingback: Cost review | Prototype Iteration

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