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Documentation / arm / sunxi / clocks.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 08:59 EST.

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
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