STM32F103RCT7 datasheet(59/130 Pages) STMICROELECTRONICS

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STM32F103xC, STM32F103xD, STM32F103xE

Electrical characteristics

Doc ID 14611 Rev 8

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

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

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

results obtained with typical external components specified in Table 24. 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).

Note:

For CL1 and CL2, it is recommended to use high-quality ceramic capacitors in the 5 pF to

15 pF range selected to match the requirements of the crystal or resonator (see Figure 23).

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.

Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray where

Cstray is the pin capacitance and board or trace PCB-related capacitance. Typically, it is

between 2 pF and 7 pF.

Caution:

To avoid exceeding the maximum value of CL1 and CL2 (15 pF) it is strongly recommended

to use a resonator with a load capacitance CL ≤ 7 pF. Never use a resonator with a load

capacitance of 12.5 pF.

Example: if you choose a resonator with a load capacitance of CL = 6 pF, and Cstray = 2 pF,

then CL1 = CL2 = 8 pF.

Table 24.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1) (2)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

CL1, CL2

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

RS = 30 kΩ

LSE driving current

VDD = 3.3 V, VIN = VSS

1.4

µA

Oscillator transconductance

µA/V

tSU(LSE)

(3)

Startup time

VDD is

stabilized

TA = 50 °C

1.5

TA = 25 °C

2.5

TA = 10 °C

TA = 0 °C

TA = -10 °C

TA = -20 °C

TA = -30 °C

TA = -40 °C

Based on characterization, not tested in production.

Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator design guide for

ST microcontrollers”.

tSU(LSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 32.768 kHz oscillation is

reached. This value is measured for a standard crystal and it can vary significantly with the crystal manufacturer

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