Tony van der Hoff  wrote:
> On 16/11/2019 23:36, Computer Nerd Kev wrote:
>> Tony van der Hoff  wrote:
>>> On 16/11/2019 10:34, Tony van der Hoff wrote:
>>>> Hi,
>>>> I'm using a headless pi1B to control some relays. One of these is
>>>> connected to GPIO14 (Pin 8). I'm finding that it jitterss rapidly on
>>>> boot, then settles down. I suspect that this behaviour is caused by its
>>>> secondary UART function, which I don't use. Is there any way to suppress
it?
>>>>
>>>
>>> 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:
>> https://www.raspberrypi.org/documentation/configuration/uart.md
>>
>> On a related note, those requiring a non-default GPIO state at
>> start-up can configure them in /boot/config.txt as described here:
>> https://www.raspberrypi.org/documentation/configuration/config-txt/gpio.md
>>
>
> 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?

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
1 and RPi Zero:
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-
ARM-Peripherals.pdf

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.

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