STM32F107RB datasheet(48/103 Pages) STMICROELECTRONICS

48 / 103 page

Electrical characteristics

STM32F105xx, STM32F107xx

48/104

Doc ID 15274 Rev 6

resonator manufacturer for more details on the resonator characteristics (frequency,

package, accuracy).

Note:

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

5 pF to 15 pF range 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.

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 23.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

C(2)

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

(3)

RS = 30 kΩ

LSE driving current

VDD = 3.3 V, VIN = VSS

1.4

µA

Oscillator Transconductance

µA/V

tSU(LSE)

(4) 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”.

The oscillator selection can be optimized in terms of supply current using an high quality resonator with small RS value for example

MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details

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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