clock control register
| HSION | Internal high-speed clock enable |
| HSIRDY | Internal high-speed clock ready flag |
| HSITRIM | Internal high-speed clock trimming |
| HSICAL | Internal high-speed clock calibration |
| HSEON | HSE clock enable |
| HSERDY | HSE clock ready flag |
| HSEBYP | HSE clock bypass |
| CSSON | Clock security system enable |
| PLLON | Main PLL (PLL) enable |
| PLLRDY | Main PLL (PLL) clock ready flag |