My past great article about what I found out happening to the GPIO pins on RESET? I was… only partially right. Some of the information turned out to be time-sensitive and has changed.
NOTE: My discussion in the old blog post.
So, let’s get this straight. The BCM2835 has a default power-on state where all GPIOs are configured as inputs. Although pull-ups are maintained in their past state even in power-down, on the next power-on, they are reset to the defaults. GPIOs 0-8 are configured as pull-up inputs and GPIOs 9-27 are configured as pull-down inputs. Then when the Raspberry Pi bootloader comes up, it initializes the pins to the default values in its Device Tree configuration. There is about one second of time between the initial reset and the first GPIO reconfiguration actions. Finally, after the Linux kernel finishes booting and loading the system, your own code will have full control.
In old versions of the Raspbian and bootloader software, the early boot environment configured the GPIO pins as high-impedance inputs without any pull up/down. This was responsible for the observed behavior of an output flicker on the GPIO ports, it was actually just a flicker of a pull-down resistor being applied and then removed. Current Raspbian and bootloader versions now set the bootloader initial value to the hardware initial value to eliminate this flicker.
The old articles:
20201025/https://raspberrypi.stackexchange.com/questions/1032/what-is-the-power-on-state-of-the-gpios
20201025/https://www.raspberrypi.org/forums/viewtopic.php?t=7443&p=93865
Note of BCM default GPIO configuration, page 102 of the BCM2835 datasheet:
20201025/DuckDuckGo raspberry pi device tree set gpio pin boot state
20201025/https://raspberrypi.stackexchange.com/questions/51479/gpio-pin-states-on-powerup#51480
Recent references about the default configuration:
20201025/DuckDuckGo raspberry pi gpio pulse during boot
20201025/https://raspberrypi.stackexchange.com/questions/75019/gpio-status-during-boot
20201025/https://discourse.osmc.tv/t/gpio-set-to-output-by-default-on-bootup/21892
20201025/https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=162242
20201025/https://github.com/raspberrypi/firmware/blob/master/extra/dt-blob.dts