3.2.1.2 System Tick Timer
The System Tick Timer's clock source is always the core clock, FCLK. This results in the
following:
• The CLKSOURCE bit in SysTick Control and Status register is always set to select
the core clock.
• Because the timing reference (FCLK) is a variable frequency, the TENMS bit in the
SysTick Calibration Value Register is always zero.
• The NOREF bit in SysTick Calibration Value Register is always set, implying that
FCLK is the only available source of reference timing.