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arm: sunxi: Add useful information about sunxi clocks

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This patch contains useful bits of information about the sunxi clocks
that may help and/or be interesting for current and future developers.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
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Emilio López 2013-02-25 11:44:28 -03:00 committed by Mike Turquette
parent e874a66977
commit 8eb896ab7a
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Documentation/arm/sunxi/clocks.txt Normal file
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Frequently asked questions about the sunxi clock system
=======================================================
This document contains useful bits of information that people tend to ask
about the sunxi clock system, as well as accompanying ASCII art when adequate.
Q: Why is the main 24MHz oscillator gatable? Wouldn't that break the
system?
A: The 24MHz oscillator allows gating to save power. Indeed, if gated
carelessly the system would stop functioning, but with the right
steps, one can gate it and keep the system running. Consider this
simplified suspend example:
While the system is operational, you would see something like
24MHz 32kHz
|
PLL1
\
\_ CPU Mux
|
[CPU]
When you are about to suspend, you switch the CPU Mux to the 32kHz
oscillator:
24Mhz 32kHz
| |
PLL1 |
/
CPU Mux _/
|
[CPU]
Finally you can gate the main oscillator
32kHz
|
|
/
CPU Mux _/
|
[CPU]
Q: Were can I learn more about the sunxi clocks?
A: The linux-sunxi wiki contains a page documenting the clock registers,
you can find it at
http://linux-sunxi.org/A10/CCM
The authoritative source for information at this time is the ccmu driver
released by Allwinner, you can find it at
https://github.com/linux-sunxi/linux-sunxi/tree/sunxi-3.0/arch/arm/mach-sun4i/clock/ccmu