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STM8AF/P6386 Datasheet(PDF) 69 Page - STMicroelectronics |
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STM8AF/P6386 Datasheet(HTML) 69 Page - STMicroelectronics |
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69 / 116 page ![]() DocID031156 Rev 1 69/116 STM8AF6388 Electrical characteristics 114 Figure 20. HSE oscillator circuit diagram HSE oscillator critical gm formula The crystal characteristics have to be checked with the following formula: Equation 1 where gmcrit can be calculated with the crystal parameters as follows: Equation 2 Rm: Notional resistance (see crystal specification) Lm: Notional inductance (see crystal specification) Cm: Notional capacitance (see crystal specification) Co: Shunt capacitance (see crystal specification) CL1 = CL2 = C: Grounded external capacitance Table 30. HSE oscillator characteristics Symbol Parameter Conditions Min Typ Max Unit RF Feedback resistor - - 220 - k Ω CL1/CL2(1) Recommended load capacitance - - - 20 pF gm Oscillator trans conductance - 5 - - mA/V tSU(HSE)(2) Startup time VDD is stabilized -2.8 - ms 1. The oscillator needs two load capacitors, CL1 and CL2, to act as load for the crystal. The total load capacitance (CLoad) is (CL1 * CL2)/(CL1 + CL2). If CL1 = CL2, Cload = CL1/2. Some oscillators have built-in load capacitors, CL1 and CL2. 2. This value is the startup time, measured from the moment it is enabled (by software) until a stabilized 24 MHz oscillation is reached. It can vary with the crystal type that is used. MSv37799V1 OSCOUT OSCIN fHSE to core CL1 RF STM8 Resonator Current control gm Rm Cm Lm Resonator CL2 CO gm gmcrit » gmcrit 2 Π × HSE f × () 2 Rm × 2Co C + () 2 = |
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