STM8AFSLASHP6388 datasheet(69/116 Pages) STMICROELECTRONICS

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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

Feedback resistor

220

CL1/CL2(1) Recommended load capacitance

Oscillator trans conductance

mA/V

tSU(HSE)(2) Startup time

VDD is

stabilized

-2.8

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

STM8

Resonator

Current control

Resonator

CL2

gm gmcrit

gmcrit

HSE

()

2Co C

()

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