STM32F042F4 datasheet(63/117 Pages) STMICROELECTRONICS

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STM32F042x4 STM32F042x6

Electrical characteristics

Figure 17. 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

obtained with typical external components specified in Table 37. 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).

Table 37. LSE oscillator characteristics (fLSE = 32.768 kHz)

Symbol

Parameter

Conditions(1)

Min(2)

Typ

Max(2)

Unit

IDD

LSE current consumption

low drive capability

0.5

0.9

µA

medium-low drive capability

medium-high drive capability

1.3

high drive capability

1.6

Oscillator

transconductance

low drive capability

µA/V

medium-low drive capability

medium-high drive capability

high drive capability

tSU(LSE)(3)

Startup time

VDDIOx is stabilized

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

ST microcontrollers”.

2. Guaranteed by design, not tested in production.

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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