| Emilio López | 8eb896a | 2013-02-25 11:44:28 -0300 | [diff] [blame] | 1 | Frequently asked questions about the sunxi clock system |
| 2 | ======================================================= |
| 3 | |
| 4 | This document contains useful bits of information that people tend to ask |
| 5 | about the sunxi clock system, as well as accompanying ASCII art when adequate. |
| 6 | |
| 7 | Q: Why is the main 24MHz oscillator gatable? Wouldn't that break the |
| 8 | system? |
| 9 | |
| 10 | A: The 24MHz oscillator allows gating to save power. Indeed, if gated |
| 11 | carelessly the system would stop functioning, but with the right |
| 12 | steps, one can gate it and keep the system running. Consider this |
| 13 | simplified suspend example: |
| 14 | |
| 15 | While the system is operational, you would see something like |
| 16 | |
| 17 | 24MHz 32kHz |
| 18 | | |
| 19 | PLL1 |
| 20 | \ |
| 21 | \_ CPU Mux |
| 22 | | |
| 23 | [CPU] |
| 24 | |
| 25 | When you are about to suspend, you switch the CPU Mux to the 32kHz |
| 26 | oscillator: |
| 27 | |
| 28 | 24Mhz 32kHz |
| 29 | | | |
| 30 | PLL1 | |
| 31 | / |
| 32 | CPU Mux _/ |
| 33 | | |
| 34 | [CPU] |
| 35 | |
| 36 | Finally you can gate the main oscillator |
| 37 | |
| 38 | 32kHz |
| 39 | | |
| 40 | | |
| 41 | / |
| 42 | CPU Mux _/ |
| 43 | | |
| 44 | [CPU] |
| 45 | |
| 46 | Q: Were can I learn more about the sunxi clocks? |
| 47 | |
| 48 | A: The linux-sunxi wiki contains a page documenting the clock registers, |
| 49 | you can find it at |
| 50 | |
| 51 | http://linux-sunxi.org/A10/CCM |
| 52 | |
| 53 | The authoritative source for information at this time is the ccmu driver |
| 54 | released by Allwinner, you can find it at |
| 55 | |
| 56 | https://github.com/linux-sunxi/linux-sunxi/tree/sunxi-3.0/arch/arm/mach-sun4i/clock/ccmu |