ST STM32F446 Series

AN4488 Rev 7 33/50
AN4488 Clocks
49
6.2 LSE OSC clock
The low-speed external clock signal (LSE) can be generated from two possible clock
sources:
• LSE user external clock (see Figure 17)
• LSE external crystal/ceramic resonator (see Figure 18)
1. “LSE crystal/ceramic resonators” figure:
To avoid exceeding the maximum value of C
L1
and C
L2
(15 pF) it is strongly recommended to use a
resonator with a load capacitance C
L
≤ 7 pF. Never use a resonator with a load capacitance of 12.5 pF.
2. “LSE external clock” and “LSE crystal/ceramic resonators” figures:
OSC32_IN and OSC32_OUT pins can be used also as GPIO, but it is recommended not to use them as
both RTC and GPIO pins in the same application.
3. “LSE crystal/ceramic resonators” figure:
The value of R
EXT
depends on the crystal characteristics. A 0 Ω resistor would work but would not be
optimal. To fine tube R
S
value, refer to AN2867 - Oscillator design guide for ST microcontrollers (Table 2)
and electrical characteristics sections in the datasheet of your product for more details.
Figure 17. LSE external clock Figure 18. LSE crystal/ceramic
resonators
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