DocID018909 Rev 11
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RM0090
Reset and clock control for STM32F42xxx and STM32F43xxx (RCC)
212
6.3.4 RCC
clock
interrupt register (RCC_CIR)
Address offset: 0x0C
Reset value: 0x0000 0000
Access: no wait state, word, half-word and byte access
Bits 7:4
HPRE:
AHB prescaler
Set and cleared by software to control AHB clock division factor.
Caution:
The clocks are divided with the new prescaler factor from 1 to 16 AHB cycles after
HPRE write.
Caution:
The AHB clock frequency must be at least 25 MHz when the Ethernet is used.
0xxx: system clock not divided
1000: system clock divided by 2
1001: system clock divided by 4
1010: system clock divided by 8
1011: system clock divided by 16
1100: system clock divided by 64
1101: system clock divided by 128
1110: system clock divided by 256
1111: system clock divided by 512
Bits 3:2
SWS:
System clock switch status
Set and cleared by hardware to indicate which clock source is used as the system clock.
00: HSI oscillator used as the system clock
01: HSE oscillator used as the system clock
10: PLL used as the system clock
11: not applicable
Bits 1:0
SW:
System clock switch
Set and cleared by software to select the system clock source.
Set by hardware to force the HSI selection when leaving the Stop or Standby mode or in
case of failure of the HSE oscillator used directly or indirectly as the system clock.
00: HSI oscillator selected as system clock
01: HSE oscillator selected as system clock
10: PLL selected as system clock
11: not allowed
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Reserved
CSSC
PLLSAI
RDYC
PLLI2S
RDYC
PLL
RDYC
HSE
RDYC
HSI
RDYC
LSE
RDYC
LSI
RDYC
w
w
w
w
w
w
w
w
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Reserv
ed
PLLSAI
RDYIE
PLLI2S
RDYIE
PLL
RDYIE
HSE
RDYIE
HSI
RDYIE
LSE
RDYIE
LSI
RDYIE
CSSF
PLLSAI
RDYF
PLLI2S
RDYF
PLL
RDYF
HSE
RDYF
HSI
RDYF
LSE
RDYF
LSI
RDYF
rw
rw
rw
rw
rw
rw
rw
r
r
r
r
r
r
r
r