STM32F042F4 datasheet(62/117 Pages) STMICROELECTRONICS

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Electrical characteristics

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DocID025832 Rev 5

High-speed external clock generated from a crystal/ceramic resonator

The high-speed external (HSE) clock can be supplied with a 4 to 32 MHz crystal/ceramic

resonator oscillator. All the information given in this paragraph are based on design

simulation results obtained with typical external components specified in Table 36. 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 startup stabilization time. Refer

to the crystal resonator manufacturer for more details on the resonator characteristics

(frequency, package, accuracy).

For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the

5 pF to 20 pF range (Typ.), designed for high-frequency applications, and selected to match

the requirements of the crystal or resonator (see Figure 17). CL1 and CL2 are usually the

same size. The crystal manufacturer typically specifies a load capacitance which is the

series combination of CL1 and CL2. PCB and MCU pin capacitance must be included (10 pF

can be used as a rough estimate of the combined pin and board capacitance) when sizing

CL1 and CL2.

Note:

For information on selecting the crystal, refer to the application note AN2867 “Oscillator

design guide for ST microcontrollers” available from the ST website www.st.com.

Table 36. HSE oscillator characteristics

Symbol

Parameter

Conditions(1)

1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.

Min(2)

Typ

Max(2)

2. Guaranteed by design, not tested in production.

Unit

fOSC_IN

Oscillator frequency

MHz

Feedback resistor

200

kΩ

IDD

HSE current consumption

During startup(3)

3. This consumption level occurs during the first 2/3 of the tSU(HSE) startup time

8.5

VDD = 3.3 V,

Rm = 30 Ω,

CL = 10 pF@8 MHz

-0.4

VDD = 3.3 V,

Rm = 45 Ω,

CL = 10 pF@8 MHz

-0.5

VDD = 3.3 V,

Rm = 30 Ω,

CL = 5 pF@32 MHz

-0.8

VDD = 3.3 V,

Rm = 30 Ω,

CL = 10 pF@32 MHz

-1

VDD = 3.3 V,

Rm = 30 Ω,

CL = 20 pF@32 MHz

-1.5

Oscillator transconductance

Startup

mA/V

tSU(HSE)(4)

4. tSU(HSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 8 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

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