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authorEmilio López <emilio@elopez.com.ar>2013-02-25 11:44:28 -0300
committerMike Turquette <mturquette@linaro.org>2013-03-27 08:35:34 -0700
commit8eb896ab7a716308698de77073a9265f584b12fc (patch)
tree4a62ae92f2287d77fb6b43028063b26b96a24455
parente874a6697710f52fa8ab29487a99034d5d96fdcc (diff)
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arm: sunxi: Add useful information about sunxi clocks
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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+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