High speed internal oscillator trimming

AN2822

8/19

Doc ID 14983 Rev 2

Figure 3.

HSI calibration flowchart

1.

The ‘abs’ function calculates absolute value.

The measurement routine results are the sum of HSI pulses during the consequential

calibration period. The number of calibration signal periods, L, used for the measurement

are given by the constant, MAX_AVERAGED_PERIOD. The more periods used for the

measurement, the lower the uncertainty of period length due to the averaging effect. The

recommended value range is 4 or 8.

prevDiff

<

actDiff

Increment

CLK_HSITRIM

End of Calibration

Yes

No

Start of Calibration

CLK_HSITRIM = MAX_TRIM_VAL

Initialization of TIM3

Measure the HSI pulses,

Calculate abs (actual – ideal),

store it in prevDiff

CLK_HSITRIM

MIN_TRIM_VAL

Yes

No

Decrement

CLK_HSITRIM

Measure the HSI pulses

abs (actual – ideal)

Store it in actDiff

prevDiff = actDiff