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NUC126
Aug. 08, 2018
Page
173
of 943
Rev 1.03
NUC12
6 S
E
RI
E
S
T
E
CH
NI
CA
L R
E
F
E
RE
NCE
MA
NUA
L
Set
HXTFDEN
To enable
HXT clock detector
HXTFIF
= 1?
System clock source =
“
HXT
”
or
“
PLL with
HXT
”
?
YES
System clock keep
original clock
NO
YES
Switch system clock to
HIRC
NO
Figure 6.3-4 HXT Stop Protect Procedure
The clock source of SysTick in Cortex
®
-M0 core can use CPU clock or external clock (SYST_CSR[2]).
If using external clock, the SysTick clock (STCLK) has 5 clock sources. The clock source switch
depends on the setting of the register STCLKSEL (CLK_CLKSEL0[5:3]). The block diagram is shown
in Figure 6.3-5.
111
011
010
001
HXT
LXT
HXT
HCLK
STCLKSEL
(CLK_CLKSEL0[5:3])
STCLK
HIRC
000
1/2
1/2
1/2
Note:
Before clock switching, both the pre-selected and newly selected clock source must be turned on and stable.
Figure 6.3-5 SysTick Clock Control Block Diagram
6.3.3
Peripherals Clock
The peripherals clock had different clock source switch setting, which depends on the different
peripheral. Please refer to the CLK_CLKSEL1, CLK_CLKSEL2 and CLK_CLKSEL3 register
description in section 6.3.7.