CSS=SLOW_CLK
Programmable Clock Register
| CSS | Programmable Clock Source Selection 0 (SLOW_CLK): SLCK is selected 1 (MAIN_CLK): MAINCK is selected 2 (PLLA_CLK): PLLACK is selected 3 (UPLL_CLK): UPLLCKDIV is selected 4 (MCK): MCK is selected  |  
| PRES | Programmable Clock Prescaler  |