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Commit 8eb896ab authored by Emilio López's avatar Emilio López Committed by Mike Turquette
Browse files

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: default avatarEmilio López <emilio@elopez.com.ar>
Acked-by: default avatarMaxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: default avatarMike Turquette <mturquette@linaro.org>
parent e874a669

Documentation/arm/sunxi/clocks.txt

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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