STMicroelectronics STM32F042C4, Manual

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STM32F042xx Electrical characteristics
94
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).
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 37. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
Symbol Parameter Conditions
(1)
Min
(2)
Typ Max
(2)
Unit
I
DD
LSE current consumption
LSEDRV[1:0]=00
lower driving capability
- 0.5 0.9
μ A
LSEDRV[1:0]= 01
medium low driving capability
- - 1
LSEDRV[1:0] = 10
medium high driving capability
- - 1.3
LSEDRV[1:0]=11
higher driving capability
- - 1.6
g
m
Oscillator
transconductance
LSEDRV[1:0]=00
lower driving capability
5 - -
μ A/V
LSEDRV[1:0]= 01
medium low driving capability
8 - -
LSEDRV[1:0] = 10
medium high driving capability
15 - -
LSEDRV[1:0]=11
higher driving capability
25 - -
t
SU(LSE)
(3)
Startup time V
DDIOx
is stabilized - 2 - s
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.
3. t
SU(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