AN3307 datasheet(32/37 Pages) STMICROELECTRONICS

32 / 37 page

Class B solution structure

AN3307

32/37

Doc ID 18186 Rev 4

5.4.4

Clock runtime self-test

The clock runtime self-test uses a similar procedure to the one used in the startup self-test.

A few underflow cycles of the SysTick timer are performed at background processing level.

The number of LSI periods counted during this period by the RTC timer gives the runtime

HSE ratio (number of HSE pulses within the interval corresponding to the LSI period) which

is stored as a Class B variable by the STL_MeasurePeriod() function called by the SysTick

interrupt service routine. The HSE value is then computed at the main processing level in

the clock run self-test. The inputs are the same as in the startup test: known HSI reference

ratio value (stored in the initial startup test phase) and the currently measured HSE ratio. If

the computed HSE value deviates from the expected 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 OK status.

Figure 18. Clock runtime self-test structure

MS18903V1

Test OK

Fail Safe routine

Fail

HSE in range

+-25 %?

Yes

HSE = ( HSI * HSI Ref Ratio ) / HSE Run Ratio

AN3307 Datasheet(PDF) 32 Page - STMicroelectronics