Class B solution structure

AN3307

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Doc ID 18186 Rev 4

interval (more than +/- 25% of its nominal value), the CPU clock source is immediately

switched back to HSI, and HSE fail status is returned. Otherwise the test returns the OK

status. In either case, the CPU clock is switched back to the default HSI source after the test

is finished.

Figure 13.

Clock frequency measurement principle

MS18599V1

Accepted

range

SysTick

Timer

(on HSI / HSE)

RTC Timer

(always on LSE)

F ~ Ref

F << Ref

F >> Ref

Run time measurement

Initial reference

measurement

Reference level

Zero level

Auto reload level

Zero level