STM32F070CB datasheet(48/83 Pages) STMICROELECTRONICS

48 / 83 page

Electrical characteristics

STM32F070CB/RB/C6/F6

48/83

DocID027114 Rev 3

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 13). 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.

Figure 13. Typical application with an 8 MHz crystal

1. REXT value depends on the crystal characteristics.

Low-speed external clock generated from a crystal resonator

The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal resonator

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

IDD

HSE current consumption

During startup(3)

8.5

VDD = 3.3 V,

Rm = 45

Ω,

CL = 10 pF@8 MHz

-0.5

VDD = 3.3 V,

Rm = 30

Ω,

CL = 20 pF@32 MHz

-1.5

Oscillator transconductance

Startup

mA/V

tSU(HSE)(4) Startup time

VDD is stabilized

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

2. Guaranteed by design, not tested in production.

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

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

Table 32. HSE oscillator characteristics

Symbol

Parameter

Conditions(1)

Min(2)

Typ

Max(2)

Unit

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