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Topic: Custom Arduino M0 board - Reset Issue (Read 2369 times) previous topic - next topic


Hello all!

I built a custom circuit based on mkrzero board.

The board works flawlessly and everything is OK.
USB is working and I can upload any program without any problem.

However, it seems that I have an issue with the reset circuit.
I first try to implement a reset circuit identical to the mkrzero. (see resetcircuit.png).

However, I couldn't start the board. After removing the capacitor, it started to work again at power on.
But now, when I press the reset button, the circuit never starts again.

Any idea of what could be the problem?
Did someone had this issue before?

Reset after programming (via USB) seems to work.

I'm not familiar with what the reset button is doing, but why powering on/off/on the circuit is working but pressing the reset button is not?

Could it be related to the fuses? I tried to program the bootloader using both Segger and Atmel-ICE.
Are the fuse automatically configured when burning the bootloader?
I'm looking for the mkrzero default fuse settings if someone has them!

I've looked at many places but couldn't find the answer.

Thank you


Comparing the ArduinoMKRZero schematic and resetcircuit.png the you linked they do not match so that could be your problem.
Don't PM me for help as I will ignore it.


That was not the problem.
But because of your comment, I checked the board again and... I found the problem.

SWCLK was pulled-down instead of pulled-up!!!!!!

The SWCLK pin must be high during reset (or else the chip goes into debugging mode). The Schematic Checklist section of the data sheet says it should be pulled up with 1k.

Thank you :-)

Hope this helps someone.


Thats interesting I was about to (attempt) to build a bare bones SAMD21, confusingly the data sheet says, page 73;

"At startup, SWCLK is internally pulled up to avoid false detection of a debugger if SWCLK is left unconnected"

So sounds like you can leave it open.

And then has the comment about the 1K resistor on page 1117;

"Note that a pull-up resistor on the SWCLK pin is critical for reliable operations"

Out of interest how did you get the bootloader onto the SAMD21 ?


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