High speed internal oscillator trimming

AN2822

14/19

Doc ID 14983 Rev 2

The combined standard uncertainty of the HSI frequency measurement is evaluated bellow.

Comparing the first and second terms we can find that a contribution caused by uncertainty,

is well below the HSI frequency.

Let’s evaluate the uncertainty caused by the finite regulation steps.

Finally, the total combined uncertainty of the HSI frequency is evaluated bellow.

Assuming that the required confidence level is 95% using the coverage factor k=2, we can

express the extended uncertainty, U, as following.

c

f

cal

f

cal

∂

∂f

HSIm

N

HSI

L

-------------

320

10

⋅

3

468.7 8.376

10

9

⋅

()

≅

==

u

c

2

f

HSIm

()

c

N

HSI

2

u

2

N

HSI

() c

f

cal

2

u

2

f

cal

()

+

468.7

8.376

10

9

⋅

+

≅

=

u

() 91.52 kHz

≅

uf

trim

()

160

10

3

⋅

23

--------------------------

46.19 kHz

≅

=

uf

HSI

()

u

2

f

trim

() u

c

2

f

HSIm

()

+

102.5 kHz

≅

=

Uk

u f

HSI

()

⋅

2

102.5 kHz

205 kHz

≅

⋅

≅

=