ST STM32F101 Series Reference Manual page 68

Reset and clock control (RCC)
Bit 6 IOPEEN I/O port E clock enable
Set and reset by software.
0: I/O port E clock disabled
1: I/O port E clock enabled
Bit 5 IOPDEN I/O port D clock enable
Set and reset by software.
0: I/O port D clock disabled
1: I/O port D clock enabled
Bit 4 IOPCEN I/O port C clock enable
Set and reset by software.
0: I/O port C clock disabled
1:I/O port C clock enabled
Bit 3 IOPBEN I/O port B clock enable
Set and reset by software.
0: I/O port B clock disabled
1:I/O port B clock enabled
Bit 2 IOPAEN I/O port A clock enable
Set and reset by software.
0: I/O port A clock disabled
1:I/O port A clock enabled
Bit 1 Reserved, always read as 0.
Bit 0 AFIOEN Alternate Function I/O clock enable
Set and reset by software.
0: Alternate Function I/O clock disabled
1:Alternate Function I/O clock enabled
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