On Sat, 6 Sep 2014 18:47:59 +0100, [email protected] wrote:
Ben Gamari <[email protected]> wrote:
> [-- text/plain, encoding 7bit, charset: ISO-8859-15, 23 lines --]
> > [email protected] writes:
> > > In the BBB System Reference Manual the P8 and P9 pinouts are
> > documented with 7 modes for each pin which provide different uses for
> > the pins.
> >
> > All of the actual software and descriptions I can find about the BBB
> > hardware just talk about the Device Tree and Device Tree overlays. I
> > have already (sort) begun to understand the Device Tree system and I
> > have actually done a (trivial) recompile of the Device Tree to change
> > one of the I2C clock frequencies.
> >
> > Do the modes in the manual have any relevance at all now? I.e. can I
> > actually start the system in a specified mode? Or are they just
> > history, or what?
> >
> If you are writing a device tree you will need to refer to the pin modes
> to configure the pinmux.
> OK, but I can't really find out *how* the PinMux/Modes are referred to
in the device tree. In fact it seems very difficult to find how the
entries in the Device Tree are mapped to the actual pins, I get the
feeling that it's actually rather simpler than it appears at first
sight but I'm still at the rather lost stage.
The other trouble is that if you look for 'how to enable the UARTs on
BBB' there are many ways of doing it:-
Edit the device tree in /boot/uboot/dtbs - this will (as I
understand it) remap things at boot time.
Create a Device Tree Template for just the things to change, this
'overlays' the Device Tree so is possible at run time (again, my
understanding)
Edit the uEnv.txt file, I assume this controls the above but it's
far from clear.
I'd love to find a document or tutorial that describes how this all
hangs together but there doesn't seem to be anything other than the
(huge) processor documentation which is vey much reference material
and (as I referred to above) various blogs etc. showing how to do one
particular thing but without any attempt to explain it.
-- Chris Green
·
Hi Chris,
The BBB SRM gives information about the BBB, but doen't go into details
about the AM3358 processor. I'd suggest reading about pinmux in the
AM335x Sitara Processors Technical Reference Manual, which can be found
on the TI website. Basically, there are far more I/O features than
pins on the processor. Eight times more signals than pins to be
exact. The modes allow selection of which one of the eight for a pin
you choose to use. The TRM will provide the details on the mode that
enables a particular function for a specific pin.
Searching for "pinmux" will find many documents and tools.
Good luck.
Larry
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