clock configuration register
| SW0 | System clock switch |
| SW1 | System clock switch |
| SWS0 | System clock switch status |
| SWS1 | System clock switch status |
| HPRE | AHB prescaler |
| PPRE1 | APB Low speed prescaler (APB1) |
| PPRE2 | APB high-speed prescaler (APB2) |
| RTCPRE | HSE division factor for RTC clock |
| MCO1 | Microcontroller clock output 1 |
| I2SSRC | I2S clock selection |
| MCO1PRE | MCO1 prescaler |
| MCO2PRE | MCO2 prescaler |
| MCO2 | Microcontroller clock output 2 |