GPIO14 jitters on boot

Well, I've figured it out for myself. For anyone finding this from a search engibe, it is due to a serial console coming up on boot. To disable:

sudo raspi-config -> Interfacing Options -> Serial -> No login Shell -> No enable serial hardware

Or, for those not using the configuration tool, by editing /boot/cmdline.txt as described here:

On a related note, those requiring a non-default GPIO state at start-up can configure them in /boot/config.txt as described here:

Thank you, Kev; That is very useful, especially the second document, of which I was unaware. It's all quite self-explanatory, except for the Alt entries. Can you (or anybody) please explain what these are?

Thanks, Tony

On Sun, 17 Nov 2019 11:23:42 +0100, Tony van der Hoff declaimed the following:

alternate-functions/

start-up conditions can never be relied upon so it is considered good practice to design hardware to minimise the afects that random noise will have.

In a few words: They enable connections to specific I/O hardware devices in the SoC. Such devices include the UARTs which allow the serial terminal function without accurate software timing. Also various other protocols, allowing them to be used without timing-accurate software, eg. SPI, I2C, PCM Audio. PWM allows for accurately controlling LED brightness, motor control (with additional circuitry), etc.

They are "alternate" functions because all the pins can also work as standard I/O under direct softare control, and there are multiple "Alt" settings because single hardware devices can be set to have their I/O connections on any of a range of pins. The actual function that a "Alt" setting corresponds to on any pin is in the datasheet for the SoC. The "BCM2835 ARM Peripherals" document (close enough to a datasheet in this case) has this info for the chip used in the RPi

Software run by the OS can always change these setting later though. eg. Linux could re-enable the UART in order to activate the serial terminal if still told to do so in cmdline.txt.

Thanks again, Kev, you have clarified much. That's one hell of a document, too much to digest for now; I've saved it for future reference.

Thanks also to the others who responded.

So far I've failed to activate the /bool/config.txt magic; I have a stanza at the end of that file: gpio=8,10,11,12=op,dh Saved, and rebooted; seems to have no effect on said pins. Presumably it expects board pin numbers, not GPIO numbers

Cheers, Tony.