How to handle inverted reset logic

[Gendozz505]

Hello,
I am working on a board that combines a SOM with an AVR MCU. The reset circuit uses inverted logic: the SOM must drive its GPIO high to pull the AVR reset pin low through a MOSFET.

I could not find a ready-made solution for this setup. Could anyone suggest how to implement this, or point me to the part of the source code where this logic can be modified?

[MCUdude]

I'll assume SOM stands for system on module? Acronyms aren't always obvious.

The reset circuit uses inverted logic: the SOM must drive its GPIO high to pull the AVR reset pin low through a MOSFET.

Fair enough. But what does this have to do with Avrdude? Are you using the module to program the AVR using Avrdude?

If your module is used to program the AVR using something like linuxspi or linuxgpio, you can invert a signal by adding a ~ in avrdude.conf:

avrdude/src/avrdude.conf.in

Lines 88 to 89 in b440362

	# # (3) To invert the polarity of a pin use a tilde: ~<num>
	# # To invert the polarity of all pins in a list use ~(<num1> [, <num2> ... ])

For instance:

programmer # raspberry_pi_gpio
    id                     = "raspberry_pi_gpio";
    desc                   = "Raspberry Pi GPIO via sysfs/libgpiod";
    type                   = "linuxgpio";
    prog_modes             = PM_ISP;
    connection_type        = linuxgpio;
    reset                  = ~22;
    sck                    = 24;
    sdo                    = 23;
    sdi                    = 18;
;

[Gendozz505]

Hi @MCUdude,

Thank you for your response.
Yes, all your assumptions are correct — the module uses linuxspi (or linuxgpio, not sure yet) to program the MCU.

If I understand correctly, I should upload my updated avrdude.conf.in file to the module and then run:

avrdude -C path/to/avrdude.conf.in

[stefanrueger]

No, avrdude.conf.in is a source file that creates acrdude.conf upon building avrdude from the sources. In your case it is sufficent to create or update your personal ~/.config/avrdude/avrdude.rc file with a new programmer entry such as for example

programmer
    id                     = "mysomgpio";
    desc                   = "My SOM GPIO via sysfs/libgpiod";
    type                   = "linuxgpio";
    prog_modes             = PM_ISP;
    connection_type        = linuxgpio;
    reset                  = ~22;
    sck                    = 24;
    sdo                    = 23;
    sdi                    = 18;
;

You can then use avrdude -c mysomgpio to program the AVR parts rigged up to your SOM.

[Gendozz505]

Thanks.
I will give it a try after I finish one another urgent task 🙏