Reset and clock control (RCC)
RM0365
127/1080
DocID025202 Rev 7
9.4 RCC
registers
for a list of abbreviations used in register descriptions.
9.4.1 Clock
control
register (RCC_CR)
Address offset: 0x00
Reset value: 0x0000 XX83 where X is undefined.
Access: no wait state, word, half-word and byte access
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Res
Res
Res
Res
Res
Res
PLL
RDY
PLLON
Res
Res
Res
Res
CSS
ON
HSE
BYP
HSE
RDY
HSE
ON
r
rw
rw
rw
r
rw
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
HSICAL[7:0]
HSITRIM[4:0]
Res
HSI
RDY
HSION
r
r
r
r
r
r
r
r
rw
rw
rw
rw
rw
r
rw
Bits 31:26 Reserved, must be kept at reset value.
Bit 25
PLLRDY:
PLL clock ready flag
Set by hardware to indicate that the PLL is locked.
0: PLL unlocked
1: PLL locked
Bit 24
PLLON:
PLL enable
Set and cleared by software to enable PLL.
Cleared by hardware when entering Stop or Standby mode. This bit can not be reset if the PLL
clock is used as system clock or is selected to become the system clock.
0: PLL OFF
1: PLL ON
Bits 23:20 Reserved, must be kept at reset value.
Bit 19
CSSON:
Clock security system enable
Set and cleared by software to enable the clock security system. When CSSON is set, the
clock detector is enabled by hardware when the HSE oscillator is ready, and disabled by
hardware if a HSE clock failure is detected.
0: Clock detector OFF
1: Clock detector ON (Clock detector ON if the HSE oscillator is ready, OFF if not).
Bit 18
HSEBYP:
HSE crystal oscillator bypass
Set and cleared by software to bypass the oscillator with an external clock. The external clock
must be enabled with the HSEON bit set, to be used by the device. The HSEBYP bit can be
written only if the HSE oscillator is disabled.
0: HSE crystal oscillator not bypassed
1: HSE crystal oscillator bypassed with external clock
Bit 17
HSERDY:
HSE clock ready flag
Set by hardware to indicate that the HSE oscillator is stable. This bit needs 6 cycles of the HSE
oscillator clock to fall down after HSEON reset.
0: HSE oscillator not ready
1: HSE oscillator ready