SKEW=PRECISE, NOREF=AVAILABLE
systick timer control register
| TENMS | system tick timer calibration value |
| SKEW | whether THE TENMS value will generate a precise 10 millisencod time or an approximation 0 (PRECISE): TENMS is considered to be precise 1 (NOT_PRECISE): TENMS is not considered to be precise |
| NOREF | whether an external reference clock is available 0 (AVAILABLE): external reference clock is available 1 (NOT_AVAILABLE): external reference clock is not available |
| EN_STCLKEN | 1 is enable STCLKEN; |