AN2867 datasheet(19/41 Pages) STMICROELECTRONICS

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DocID15287 Rev 9

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AN2867

Pierce oscillator design

3.8.2

Measurement methodology

To measure the oscillator negative resistance, a resistance is added in series with the

resonator as described in Figure 8.

The oscillator negative resistance is the value of smallest series resistance that will prevent

the oscillator from starting up successfully.

In practice, this is achieved by conducting several experiments in which the value of the

series resistance is slightly increased compared to the previous experiment. This sequence

of experiments should stop when the oscillator is not able to start up correctly. This allows

determining the oscillator negative resistance which is equal to the value of the added series

resistance.

Figure 8. Negative resistance measurement methodology description

3.8.3

Safety factor for STM32 and STM8 oscillators

Table 4 gives a summary of the safety factors for the oscillators embedded in STM32 and

STM8 microcontrollers. It should be noted that for the LSE oscillator, the oscillation is

considered safe for a safety factor higher than or equal to x3, while for the HSE oscillator,

the oscillation is considered safe starting from a safety factor higher than or equal to x5.

Table 4. Safety Factor (Sf) for STM32 and STM8 oscillators(1)

1. Safe and very safe oscillations are shown in green while not safe oscillation is show in orange.

Safety Factor (Sf)

Assurance level

HSE

LSE

Sf ≥ 5

Safe

Very Safe

3 ≤ Sf < 5

Not Safe

Safe

Sf < 3

Not Safe

AN2867 Datasheet(PDF) 19 Page - STMicroelectronics