STM8AF528ATAY datasheet(68/114 Pages) STMICROELECTRONICS

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Electrical characteristics

STM8AF52/62xx, STM8AF51/61xx

68/114

DocID14395 Rev 10

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 34. 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.

OSCOUT

OSCIN

fHSE to core

CL1

CL2

STM8A

Resonator

Current control

Resonator

gm gmcrit

gmcrit

HSE

()

2Co C

()

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