STLUX datasheet(97/126 Pages) STMICROELECTRONICS

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STLUX

Electrical characteristics

126

Figure 20. HSE external clock source

HSE crystal/ceramic resonator oscillator

The HSE clock can be supplied with a 1 to 24 MHz crystal/ceramic resonator oscillator. All

the information given in this paragraph is based on characterization results with specified

typical external components. In the application, the resonator and the load capacitors have

to be placed as close as possible to the oscillator pins in order to minimize output distortion

and start-up stabilization time. Refer to the crystal resonator manufacturer for more details

(frequency, package, accuracy, etc.).

Table 78. HSE crystal/ceramic resonator oscillator

Symbol

Parameter

Conditions

Min. Typ.

Max.

Unit

fHSE

External high speed oscillator

frequency

116(1)

1. In case fHSE is configured as a direct clock for the SMED logic the maximum frequency can be 24 MHz.

MHz

Feedback resistor

220



CL1, CL2(2)

2. 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.

Recommended load capacitance(3)

3. The oscillator selection can be optimized in terms of supply current using a high quality resonator with

small Rm value.

IDD(HSE) HSE oscillator power consumption

6 (startup)

2 (stabilized)

Oscillator transconductance

mA/V

tSU(HSE)(4)

4. tSU(HSE) is the start-up time measured from the moment it is enabled (by software) to a stabilized 16 MHz

oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly

with the crystal manufacturer.

Startup time

VDD is stabilized

2.8

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