Documentation/arm/sunxi/clocks.txt

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