Why does my led lights a litlle at boot ?

Official Hardware Mega GIGA R1 WiFi
1

Hi everybordy !

During my tests, I noticed that a LED connected to pin 7 lights up slightly just after the reset.

Even with the simplest of sketches like "Blink", the LED lights up slightly before the program actually starts. I haven't noticed this problem on other digital pins for the moment.

I don't see an explanation for the Giga pinout either. This pin does not appear to be used in any way by the system (I2C, UART or otherwise).

What could be the reason for this slight illumination before the real start of the program, including with the simplest sketch "Blink"?

Thanks!

2

Arduino pin 7 is PB4 on STM32, this is a GPIO but also a system pin: JTRST (JTAG Test Reset). During boot process, the STM32 checks if a JTAG connection is present, so this pin might go to high briefly during booting of the STM32.

Hope this helps.

best regards
Herbert

3

Thank you for your quick answer !

And there is no way to "disable" this ?

I tried this whitout success:

Thanks!

4

Unfortunately no. SYS pins cannot be disabled. Apart from this "interesting" boot behavior the pin can be used in a normal way as long as you do not debug via JTAG.

5

Hi @formulanay,

We have updated our documentation about this pin. You can have a look here: