RM0367 Rev 7 187/1043
RM0367 Reset and clock control (RCC)
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7.3 RCC registers
Refer to Section 1.2 for a list of abbreviations used in register descriptions.
7.3.1 Clock control register (RCC_CR)
Address offset: 0x00
System Reset value: 0b0000 0000 00XX 0X00 0000 0011 0000 0000 where X is undefined
Power-on reset value: 0x0000 0300
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. RTCPRE[1:0]
CSSHS
EON.
HSE
BYP
HSE
RDY
HSE
ON
r rw rwrwrwrw r rw
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Res. Res. Res. Res. Res. Res.
MSI
RDY
MSION Res. Res.
HSI16
OUTEN
HSI16
DIVF
HSI16
DIVEN
HSI16
RDYF
HSI16K
ERON
HSI16
ON
rrw rwrrwrrwrw
Bits 31:26 Reserved, must be kept at reset value.
Bit 25 PLLRDY: PLL clock ready flag
This bit is set by hardware to indicate that the PLL is locked.
0: PLL unlocked
1: PLL locked
Bit 24 PLLON: PLL enable bit
This bit is 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 23:22 Reserved, must be kept at reset value.
Bits 21:20 RTCPRE[1:0] RTC/LCD prescaler
These bits are set and reset by software to obtain a 4 MHz clock from HSE. This prescaler
cannot be modified if HSE is enabled (HSEON = 1).These bits are reset by a power -on
reset,. Their value is not modified by a system reset.
00: HSE is divided by 2 for RTC/LCD clock
01: HSE is divided by 4 for RTC/LCD clock
10: HSE is divided by 8 for RTC/LCD clock
11: HSE is divided by 16 for RTC/LCD clock
Bit 19 CSSHSEON: Clock security system on HSE enable bit
This bit is set by software to enable the clock security system (CSS) on HSE. This bit is "set
only" (disabled by system reset). When CSSHSEON is set, the clock detector is enabled by
hardware when the HSE oscillator is ready, and disabled by hardware if an oscillator failure
is detected.
0: Clock security system OFF (clock detector OFF)
1: Clock security system ON (clock detector ON if HSE oscillator is stable, OFF otherwise)
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